Effects of dietary-induced hyperparathyroidism on the parathyroid hormone-receptor-adenylate cyclase system of canine kidney. Evidence for postreceptor mechanism of desensitization.

The present studies were designed to examine the consequences of chronic mild elevations of endogenous parathyroid hormone (PTH) in vivo on the PTH receptor-adenylate cyclase system of canine kidney cortex. Hyperparathyroidism was induced in normal dogs by feeding a diet low in calcium, high in phosphorus to the animals for a period of 6-9 wk. This maneuver resulted in a two to threefold increase in the plasma levels of carboxy-terminal immunoreactive PTH. This degree of hyperparathyroidism is similar to that seen in patients with hyperparathyroidism and normal renal function. After 6-9 wk on the diet the animals were killed and basolateral renal cortical membranes prepared for the study of the PTH receptor-adenylate cyclase system in vitro. The dietary hyperparathyroidism resulted in desensitization of the PTH-responsive adenylate cyclase (Vmax 3,648 +/- 654 pmol cyclic (c)AMP/mg protein per 30 min in hyperparathyroid animals vs. 5,303 +/- 348 in normal controls). The Kact (concentration of PTH required for half-maximal enzyme activation) was unchanged. However, PTH receptor binding (125I-norleucyl8-norleucyl18-tyrosinyl34, 125I[Nle8, Nle18, Tyr34] bPTH (1-34) NH2 as radioligand) was not different in the two groups of animals. Thus, dietary hyperparathyroidism resulted in an uncoupling of the PTH receptor-adenylate cyclase system. This defect was not corrected by guanyl nucleotides in vitro, and the effects of guanyl nucleotides on PTH binding and enzyme activation appeared normal. NaF-stimulated enzyme activity was reduced in the hyperparathyroid animals (8,285 +/- 607 pmol cAMP/mg protein per 30 min vs. 10,851 +/- 247 in controls). These data indicate that desensitization of the PTH-responsive adenylate cyclase system of canine kidney as a result of mild chronic elevations of endogenous PTH is due to a postreceptor defect, demonstrable by NaF activation, not corrected by guanyl nucleotides, leading to abnormal PTH-receptor adenylate cyclase coupling.     

Effect of parathyroid hormone fragments on calcium transport in toad bladder

Parathyroid hormone (PTH) has a variety of biologic effects which are both dependent and independent on activation of adenylate cyclase. We studied the effects of intact PTH and PTH fragments on water flow and Ca transport in isolated toad bladder sacs. As reported previously by us, PTH (1-84) significantly stimulated basal water flow in isolated toad bladder sacs. Synthetic PTH 1-34, 44-68, 53-84 and 65-84 (1 microgram/ml) had no effect on basal water flow after a 60-min incubation period. Intact PTH (1-84) and synthetic 1-34 PTH significantly inhibited both arginine-vasopressin and cyclic AMP-stimulated water flow. Synthetic PTH 44-68, 53-84 and 65-84 (1 microgram m/ml) had no effect on arginine-vasopressin or cyclic AMP-stimulated water flow after a 60-min incubation. Intact PTH (1-84) and synthetic 1-34 PTH significantly stimulated 45Ca uptake without affecting 45Ca efflux. Synthetic PTH 44-68, 53-84 and 65-84 had no effect on either 45Ca uptake or 45Ca efflux. Although these results suggest that the intact hormone is required for the maximal effect of PTH on water flow, substantial activity resides in the amino terminal fragment of the hormone. No activity per se resides in the carboxy terminal portion of the hormone as regards water flow. Alterations in Ca transport appear to mediate the effect of PTH on water flow. These effects are independent of activation of adenylate cyclase because these hormones also inhibit cyclic AMP-stimulated water flow.     

Human renal carcinoma cells produce hypercalcemia in the nude mouse and a novel protein recognized by parathyroid hormone receptors

When grown in nude mice, cultured renal carcinoma cells from a hypercalcemic patient produced marked hypercalcemia that was reversed by resection of tumor. Conditioned medium from this cell line contained a protein with activity in a renal adenylate cyclase bioassay for parathyroid hormone (PTH) which was blocked by the competitive PTH antagonist [8norleucyl, 18norleucyl, 34tyrosinyl]bPTH (3-34)amide. However, the biologically active protein was eluted from gel filtration columns as a larger molecular size component that PTH and was not recognized by any of four region-specific PTH antisera. The properties of this factor resemble those of the postulated PTH-like substance(s) in humoral hypercalcemia of malignancy.     

Effects of choroid plexus peptide IIF on adenylate cyclase and 3',5'-cyclic adenosine monophosphate in adipose tissue.

Two hypophyseal lipolytic peptides, adrenocorticotropin (ACTH) and beta-melanocyte-stimulating hormone (beta-MSH), and the extrhypophyseal lipolytic peptide IIF, were compared with regard to their effects on free fatty acid production and 3',5'-cyclic adenosine monophosphate (cAMP) concentration in isolated rabbit and rat adipose tissue, and on adenylate cyclase activity in the tissue homogenates. ACTH at concentrations of 0.01 mug/ml or more increased lipolysis and cAMP levels in both tissues. beta-MSH at concentrations of 0.001 mug/ml or more increased lipolysis and cAMP in the rabbit tissue, but a concentration of 10 mug/ml did not stimulate lipolysis and did not alter nucleotide concentration in the rat tissue. Peptide IIF at 0.01 mug/ml or more stimulated lipolysis in rabbit adipose tissue and caused an accumulation of cAMP. A concentration of 100 mug/ml failed to stimulate free fatty acid production in the rat tissue and the cAMP level was also unaffected. In a medium containing 7.6 mEq/l of Mg++ and no Ca++, ACTH at 0.1 mug/ml or more stimulated adenylate cyclase activity in both rabbit and rat adipose homogenates by 6- to 12-fold. This effect was inhibited when Mg++ was replaced by Ca++, Na+ or K+. beta-MSH stimulated adenylate cyclase in rabbit, but not in rat, adipose homogenate in Mg++-containing incubation midium; again, the effect on rabbit adenylate cyclase was suppressed when Mg++ was replaced by Ca++, Na+ or K+. Peptide IIF failed to influence adenylate cyclase in the rabbit tissue homogenate in the Mg++-containing, Ca++-free medium; but when the medium contained 7.6 mEq/l of Ca++ in place of Mg++, 0.1 mug/ml or more of IIF caused a 4- to 15-fold increase in cyclase activity. IIF did not affect cyclase in the rat tissue homogenate in the presence or absence of Ca++. The data are consistent with the conclusion that extrahypophyseal lipolytic peptide IIF, as well as hypophyseal peptides ACTH and beta-MSH, accelerates lipolysis in susceptible adipocytes by stimulating adenylate cyclase to produce cAMP. The effect of IIF on cyclase requires the presence of exogenous Ca++; that of ACTH and beta-MSH requires exogenous Mg++.     

Differential regulation of right and left ventricular beta-adrenergic receptors in newborn lambs with experimental cyanotic heart disease.

To determine whether chronic hypoxemia secondary to an intracardiac right-to-left shunt alters regulation of the myocardial beta-adrenergic receptor/adenylate cyclase system, we produced chronic hypoxemia in nine newborn lambs by creating right ventricular outflow obstruction and an atrial septal defect. Oxygen saturation was reduced to 65-74% for 2 wk. Eight lambs served as normoxemic controls. beta-receptor density (Bmax) and ligand affinity (KD) were determined with the radio-ligand [125I]iodocyanopindolol and adenylate cyclase activity determined during stimulation with isoproterenol, sodium fluoride (NaF), and forskolin. During chronic hypoxemia, Bmax decreased 45% (hypoxemic, 180.6 +/- 31.5 vs. control, 330.5 +/- 60.1 fmol/mg) in the left ventricle (exposed to hypoxemia alone) but was unchanged in the right ventricle (exposed to hypoxemia and pressure overload). KD was not different from control in either ventricle. Left ventricular isoproterenol-stimulated adenylate cyclase activity was decreased by 39% (30.0 +/- 4.3% increase vs. 44.1 +/- 9.5% increase) whereas right ventricular adenylate cyclase activity was unchanged. Stimulation of adenylate cyclase with NaF or forskolin was not different from control in either ventricle. Circulating epinephrine was increased fourfold whereas circulating and myocardial norepinephrine were unchanged. These data demonstrate a down-regulation of the left ventricular beta-adrenergic receptor/adenylate cyclase system during chronic hypoxemia secondary to an intracardiac right-to-left shunt.     

An investigation of the antinociceptive activity of calcitonin gene-related peptide alone and in combination with morphine: correlation to 45Ca++ uptake by synaptosomes

A 5- or 30-min incubation of synaptosomes with calcitonin gene-related peptide (CGRP) (10(-6), 10(-7), 10(-8) M) produced increases in 45Ca++ uptake upon depolarization of the synaptosomes, whereas a naloxone-reversible decrease in 45Ca++ uptake was seen after a 1-hr incubation with CGRP. Morphine-induced (10(-6) M) decreases in 45Ca++ uptake were reversed by simultaneous 5-min incubation of synaptosomes with CGRP (10(-6) M) and were enhanced by a 1-hr preincubation of the synaptosomes with CGRP (10(-6) M). CGRP produced naloxone-reversible antinociception in both the p-phenylquinone and hot-plate tests at 1 hr after i.c.v. administration, a time which corresponds to the peak CGRP-induced decrease in 45Ca++ uptake. CGRP (0.02 microgram through 40 micrograms i.c.v.) failed to produce antinociception in the tail-flick test. However, 30-min pretreatment of mice with CGRP (2 micrograms i.c.v.), a time point corresponding to a CGRP-induced increase in 45Ca++ uptake in vitro, significantly antagonized morphine-induced antinociception in the tail-flick test. A 1-hr pretreatment of mice with CGRP (2 micrograms i.c.v.), a time point in which a CGRP-induced decrease in 45Ca++ uptake was observed, slightly potentiated morphine in the tail-flick and hot-plate tests, but not in the p-phenylquinone test. The effects of CGRP on Ca++ uptake are suggestive of, but do not entirely predict, its activity in vivo and in vitro.     

Rapid development of renal resistance to low doses of synthetic bovine parathyroid hormone fragment 1-34. Dissociation of urinary cyclic adenosine monophosphate, phosphaturic, and calciuric responses.

The designing of parathyroid hormone (PTH)-renal dose-response studies in human beings is complicated by the possibility of rapid homologous receptor down-regulation, a phenomenon that is clearly shown to occur in vitro. Large amounts of PTH given to human subjects as serial injections or prolonged infusions cause decreased urinary 3',5'-cyclic adenosine monophosphate (cAMP) responses to subsequent PTH doses, but it is uncertain whether lower doses given over shorter periods similarly cause renal tachyphylaxis to PTH action. Thus, in seven water-loaded adults, we infused in ascending order 10, 30, 75, 150, and 300 U of synthetic bovine PTH fragment 1-34 (bPTH 1-34) per 70 kg body wt over 15 min on widely separated days ("separate day administration"). On another day, each subject received all five 15-min doses in ascending order at 75-min intervals ("single day administration"). Urine collection intervals were control, 0-30 min (including the PTH infusion), and 30-60 min. Peak nephrogenous cAMP (NcAMP, nmol/100 ml glomerular filtrate) response was linearly related to the dose of PTH (separate day study, r = 0.94, P less than 0.001; single day study, r = 0.88, P less than 0.001). However, the slope of NcAMP responses plotted against PTH dose for the single day study was only 36% of that derived from separate day administration of the same PTH doses (P less than 0.001). After only 40 U (10 + 30) of bPTH 1-34/70 kg, the NcAMP response to 75 U was reduced 44%, and the effect of 300 U/70 kg, when given as the last of the sequential single day infusions, was 64% less than the response to 300 U of bPTH 1-34 given alone (P less than 0.001). The phosphaturic response (fractional excretion of phosphorus, FEP [percent]) was also linearly related to bPTH 1-34 dose, but combined administration of the PTH infusions on one day increased FEP at each dose identically with the effects of separate day administration. A transient, dose-related, early hypercalciuric response to bPTH 1-34 also occurred, and was of equal magnitude in both protocols. These studies demonstrate that significant blunting of the NcAMP response to bPTH 1-34 occurs rapidly and follows brief exposure to relatively low doses of hormone. In contrast, there is no effect of recent PTH administration on the phosphaturic and early hypercalciuric actions of bPTH 1-34. This seeming dissociation of PTH effects makes unclear the physiologic importance of PTH-induced cAMP tachyphylaxis in the regulation of final PTH actions. In any case, studies of NcAMP responses in which the occurrence of tachyphylaxis would be undesirable should be designed to avoid prolonged or closely spaced administrations of the hormone.     

Modulation of levels of free calcium within synaptosomes by organochlorine insecticides

Effects of the organochlorine insecticides chlordecone, mirex, 1-(2-chlorophenyl)-1-(4-chlorophenyl)-2,2,2-trichloroethane and 1,1-bis(4-chlorophenyl)-2,2,2-trichloroethane on free intrasynaptosomal Ca2+ [( Ca++]i), synaptosomal 45Ca uptake and synaptosomal plasma and mitochondrial membrane potentials in vitro were studied. Chlordecone (10-50 microM) increased [Ca++]i from the resting level of 370 nM in a dose- and time-dependent manner to above 1.5 microM. This took place in the presence of 1 mM extrasynaptosomal Ca++ but not in nominally Ca++-free medium. Verapamil, a voltage sensitive Ca++ channel blocker, inhibited the initial increase of [Ca++]i caused by chlordecone, by 40%. Chlordecone also elevated [Ca++]i in synaptosomes in which mitochondrial Ca++ uptake had been abolished by valinomycin. Chlordecone depolarized partially the synaptosomal plasma membrane and, to a lesser extent, the potential of mitochondria within synaptosomes. However, chlordecone appeared to inhibit synaptosomal K+-stimulated and unstimulated 45Ca++ uptake by 20 to 30%. Inasmuch as chlordecone also stimulated release of 45Ca++ and the fluorescent dye fura-2 from preloaded synaptosomes, the apparent inhibition of uptake might be due to lysis of some synaptosomes by chlordecone. The effect of chlordecone on [Ca++]i decreased when the total amount of tissue in incubations was increased. [Ca++]i was only elevated marginally by mirex at the same concentration range. The results suggest that chlordecone increases free intrasynaptosomal Ca++ mainly by increasing influx of extrasynaptosomal Ca++. The principal mechanism appears to be a nonspecific leakage of Ca++ through the plasma membrane but some Ca++ may pass through voltage-sensitive Ca++ channels due to chlordecone-induced membrane depolarization.     

Tuberoinfundibular peptide (7-39) [TIP(7-39)], a novel, selective, high-affinity antagonist for the parathyroid hormone-1 receptor with no detectable agonist activity

The parathyroid hormone (PTH)-1 receptor mediates the pathophysiological effects of PTH in hyperparathyroidism and PTH-related protein (PTHrP) in humoral hypercalcemia of malignancy. A PTH1 receptor antagonist may be of therapeutic utility in these disorders. We recently identified a novel antagonist, tuberoinfundibular peptide (7-39) [TIP(7-39)], derived from the likely endogenous ligand for the PTH2 receptor TIP39. In this study its in vitro profile is evaluated and compared with that of [D-Trp(12),Tyr(34)]bPTH(7-34) and PTHrP(7-34), representing the two previously known structural classes of PTH1 receptor antagonists. TIP(7-39) binds with higher affinity (6.2 nM) to the PTH1 receptor than [D-Trp(12),Tyr(34)]bPTH(7-34) (45 nM) and PTHrP(7-34) (65 nM) and displays a 5.5-fold greater PTH1/PTH2 receptor selectivity. TIP(7-39) does not stimulate cAMP accumulation via the PTH1 receptor [in a sensitive assay that detects the activity of the weak partial agonist [Nle(8,18),Tyr(34)]bPTH(3-34)] and does not increase intracellular calcium. Schild analysis for TIP(7-39) was consistent with purely competitive antagonism of PTH(1-34)'s stimulation of cAMP accumulation (slope = 0.99 +/- 0.24). The pK(B) for TIP(7-39) (7.1 +/- 0.3) was higher than that for [D-Trp(12),Tyr(34)]bPTH(7-34) (6.5 +/- 0.0) and PTHrP(7-34) (6.0 +/- 0.1). Binding of (125)I-TIP(7-39) to the PTH1 receptor could be measured (K(D) = 1.3 +/- 0.1 nM, B(max) = 1.3 +/- 0.1 pmol/mg), whereas binding of (125)I-[Nle(8,18),D-Trp(12),Tyr(34)]bPTH(7-34) could not be detected. Kinetic analysis indicated that (125)I-TIP(7-39) dissociates much more slowly (t(1/2) = 14 min) than [D-Trp(12),Tyr(34)]bPTH(7-34) (13 s) and PTHrP(7-34) (9 s). The novel antagonist TIP(7-39) therefore displays a more favorable in vitro pharmacological profile than antagonists derived from PTH and PTHrP and may be useful for demonstrating the utility of PTH1 receptor antagonism in the treatment of hypercalcemia.     

Effects of parathyroid hormone on cytosolic free calcium concentration in individual rabbit connecting tubules.

PTH stimulates active Ca reabsorption in isolated perfused rabbit kidney connecting tubules (CNTs). The existence of PTH-sensitive adenylate cyclase and the reproduction of increased epithelial Ca transport by dibutyryl-cAMP suggest that cAMP is the mediator. Accordingly, we studied the effects of PTH and 8-bromoadenosine 3',5'-cAMP (8-Br-cAMP) on cytosolic free calcium concentration [( Ca2+]i) in individual rabbit CNTs. [Ca2+]i was estimated by continuous epifluorescence microscopy of single fura-2-loaded tubules during dual wave-length excitation. In nonperfused controls at 37 degrees C, [Ca2+]i decreased with time. In contrast to vehicle controls, synthetic bovine (1-34) PTH (0.1 nM) increased [Ca2+]i within 4 min, produced a maximal effect in 7.2 min, and sustained its effect for at least 2 min after washout. 8-Br-cAMP (1 mM) mimicked the effect of PTH, but with an earlier onset of action. To test the hypothesis that lumen Ca is the predominant source of the rise in [Ca2+]i, we studied singly perfused CNTs. In the absence of bath and lumen Ca, PTH elicited no rise in [Ca2+]i, implying that intracellular Ca stores are not the major source. In contrast, there was a rise when Ca was replenished in both media. In the continuous presence of bath Ca, lumen Ca was estimated to contribute 65% of the total rise in [Ca2+]i in response to PTH when it was first deleted and then replenished. However, when the sequence of lumen Ca manipulation was reversed, the contributions by lumen and bath Ca were found to be essentially equal. We conclude (a) at a physiologic concentration, PTH increases [Ca2+]i in rabbit CNTs, (b) 8-Br-cAMP mimics this action, implicating cAMP as a second messenger, and (c) the PTH-stimulated rise in [Ca2+]i depends importantly on both bath and tubular luminal fluid Ca.     

Inhibitory effects of amphetamine on potassium-stimulated release of [3H]dopamine from striatal slices and synaptosomes

Amphetamine, 10(-7) M or greater, evoked the release of [3H]dopamine ([3H]DA) and inhibited subsequent K+-evoked [3H]DA release from striatal synaptosomes superfused at a flow rate (1 ml/min) that prevented reuptake. Amphetamine inhibited the K+-evoked release of [3H]DA to a lesser extent in striatal slices or in synaptosomes superfused at a flow rate (0.35 ml/min) that allowed reuptake. The observed decrease in amphetamine inhibition of K+-evoked release was primarily due to amphetamine blocking [3H]DA reuptake. Interneuronal interactions may account for some of the inhibitory effects of amphetamine on K+-evoked release in the slice. Inhibition of K+-evoked release from either slices or synaptosomes was still evident when 10(-6) M amphetamine was removed from the superfusion buffer and the spontaneous release had returned to control levels. The presence of Ca++ during amphetamine exposure was required for subsequent inhibition of K+-evoked release in synaptosomes. Amphetamine in the presence of Ca++ did not affect the subsequent release of [3H]DA evoked by the Ca++ ionophore, A23187. Therefore, amphetamine inhibition of the K+-evoked release of [3H]DA cannot be explained by prior depletion of Ca++-releasable pools. Nifedipine, 1 microM, failed to block either the Ca++-dependent release of [3H]DA or the inhibition of K+-evoked release by amphetamine. However, 1 mM cobalt inhibited the Ca++-dependent release of [3H]DA by amphetamine and antagonized the inhibition of K+-evoked release after amphetamine exposure. This suggests that amphetamine may open voltage-dependent Ca++ channels sensitive to cobalt but not nifedipine. Amphetamine may desensitize these voltage-dependent Ca++ channels and inhibit their activation by K+ depolarization.     

A simple method to determine adenylate cyclase activity in isolated single nephron segments by radioimmunoassay for succinyl adenosine 3',5'-cyclic monophosphate.

A simple method to determine adenylate cyclase activity in isolated single nephron segments is described. Segments of the proximal convoluted tubule or the cortical collecting tubule were isolated from rabbit kidney slices pretreated with collagenase. After the tubule membranes were made permeable by adding hypotonic medium and freezing-thawing, each sample was incubated at 30 degrees C for 30 min in a medium containing ATP and theophylline. Generated cAMP was succinylated and served for radioimmunoassay. Addition of the incubation medium did not interfere the radioimmunoassay. Recovery of added cAMP was 96%. In the proximal convoluted tubule, either 8 mM NaF or 1 U/ml parathyroid hormone (PTH) markedly stimulated adenylate cyclase activity, but 1 mU/ml arginine vasopressin (AVP) did not. By contrast, in the cortical collecting tubule, either 8 mM NaF or 1 mU/ML AVP markedly stimulated adenylate cyclase activity, but 1 U/ml PTH did not. These data imply that this method is sensitive enough to detect either specific or nonspecific response of adenylate cyclase activity in single nephron segments.     

K+ channel and adenylate cyclase involvement in regulation of Ca2+-evoked release of [3H]dopamine from synaptosomes

In superfused striatal synaptosomes, previously unexposed to Ca2+ during isolation and superfusion, 1.25 mM Ca2+ evokes the release of [3H]dopamine. This Ca2+-evoked release is produced without elevating K+ (4.5 mM) before or after Ca2+ exposure, can be blocked by the Na+ channel antagonist tetrodotoxin, and modulated by dopamine (D2) receptor agonists and antagonists. We now present evidence that functional K+ channels regulate Ca2+-evoked [3H]dopamine release and may be necessary for the dopamine (D2) modulation of this release. The K+ channel blocker tetraethyl ammonium (TEA) could partially prevent D2 agonist (LY-171555) inhibition of Ca2+-evoked release in both olfactory tubercle and striatal synaptosomes. Another K+ channel blocker, 4-aminopyridine, also partially blocked dopamine (D2) agonist inhibition of release. When both 5 mM tetraethyl ammonium and 0.1 mM 4-aminopyridine were employed, by, dopamine (D2) inhibition of Ca2+-evoked [3H]dopamine release was prevented. However, with both K+ channel blockers present, only the initial portion of the release could be blocked by tetrodotoxin. These results are consistent with what might be expected if K+ channels were linked to dopamine (D2) receptors. In additional experiments we found that stimulation of adenylate cyclase by 1 microM forskolin with 0.25 mM 3-isobutyl-1-methylxanthine present potentiated Ca2+-evoked [3H]dopamine release but that this combination did not affect dopamine (D2) inhibition of [3H]dopamine release. Furthermore, although the protein alkylator n-ethylmaleimide could block dopamine (D2) inhibition of release, pertussis toxin, a specific inactivator of the inhibitory protein regulating adenylate cyclase, had little effect on dopamine (D2) inhibition. Therefore, dopamine (D2) inhibition of dopamine release may not be coupled to adenylate cyclase activity.     

Effect of forskolin on cytosolic Ca++ level and contraction in vascular smooth muscle

The effects of forskolin, an activator of adenylate cyclase, on cytoplasmic Ca++ level ([Ca++]cyt) measured simultaneously with muscle tension using fura-2-Ca++ fluorescence were examined in isolated smooth muscle of rat aorta. Forskolin decreased muscle tension and [Ca++]cyt in resting aorta whereas both norepinephrine and high K+ solution produced sustained increase in muscle tension and [Ca++]cyt. Addition of forskolin during the sustained contractions decreased muscle tension more strongly than [Ca++]cyt. Norepinephrine-induced contraction was more sensitive to forskolin than high K+-induced contraction. The inhibitory effect of forskolin was attenuated when the concentration of norepinephrine or K+ was increased. Cumulative addition of norepinephrine or K+ induced a concentration-dependent increase in both [Ca++]cyt and muscle tension and a positive [Ca++]cyt-tension correlation was observed. In the presence of 0.1 microM forskolin, the norepinephrine-induced increments in [Ca++]cyt and muscle tension were inhibited without changing the [Ca++]cyt-tension relationship. In the presence of a higher concentration (1 microM) of forskolin, muscle tension was inhibited more strongly with only a small additional decrease in [Ca++]cyt resulting in a shift of the [Ca++]cyt-tension relationship. Norepinephrine induced transient increments in [Ca++]cyt and muscle tension in Ca++-free solution and forskolin inhibited these changes. These results suggest that forskolin has concentration-dependent inhibitory effects on vascular contractility to decrease [Ca++]cyt at lower concentrations and to decrease the sensitivity of contractile elements to Ca++ at higher concentrations.     

Octimibate inhibition of platelet aggregation: stimulation of adenylate cyclase through prostacyclin receptor activation

Octimibate inhibited ADP- and collagen-induced platelet aggregation in human, rabbit and rat platelet-rich plasma. Washed human platelets treated with octimibate had elevated cyclic AMP (cAMP) levels and cAMP-dependent protein kinase activity. When whole platelets were incubated with radiolabeled phosphate, octimibate produced an increase in the phosphorylation of platelet proteins with relative molecular weights of 22, 26, 50 and 80 kilodaltons. This pattern of protein phosphorylation is identical to that observed when the platelets were treated with forskolin, phosphodiesterase inhibitors or other compounds that elevate platelet cAMP levels. Octimibate also inhibited the rise in intracellular Ca++ caused by thrombin, as measured using Fura-2-loaded platelets, which is consistent with octimibate's ability to elevate platelet cAMP levels. When isolated platelet plasma membranes were treated with octimibate, adenylate cyclase activity was stimulated, reaching maximal activation at 1 microM octimibate. (The maximal activation of adenylate cyclase observed with octimibate is 70-75% of that observed with 10 microM PGE1.) This stimulation of platelet adenylate cyclase activity was enhanced by GTP. Octimibate competed for radiolabeled prostaglandin E1 and lloprost binding to isolated platelet membranes at submicromolar concentrations, but did not compete with radiolabeled prostaglandin D2 binding. These studies suggest that octimibate inhibits platelet aggregation by activating platelet adenylate cyclase through stimulation of platelet prostacyclin receptors.     

Adaptive mechanisms of striatal D1 and D2 dopamine receptors in response to a prolonged reserpine treatment in mice

Mice receiving reserpine (1 mg/kg/day) during 5 days develop behavioral supersensitivity. To study the possible molecular correlates of these adaptive changes we compared binding parameters of D1 and D2 receptors and adenylate cyclase activity in striata from normal and reserpinized mice. Saturation curves using [3H]SCH 23390 showed no changes in maximum binding capacity (Bmax) or Kd of striatal D1 receptors taken from control or 5 days reserpine-treated mice. However, [3H]spiperone saturation curves showed a 31% increase in D2 receptors Bmax with no changes in Kd. Dopamine competition of [3H]SCH 23390 and [3H]spiperone binding in mouse striatum was also performed. Analysis of data by LIGAND showed that dopamine recognizes two subpopulations for D1 and for D2 receptors. The proportion of receptors in the high affinity state (D1high and D2high) were increased in reserpine-treated animals. The addition of 100 microM GTP produced a complete conversion of D1high and D2high receptors into their low-affinity states in striata from control and reserpinized mice. Five days of reserpine treatment increased basal adenylate cyclase activity of mouse striatum in the presence of Mn++ or Mg++ ions. Concentration curves with dopamine, NaF or forskolin revealed shifts to the left and higher maximum responses without changes in EC50 values in striata from reserpinized mice. Thus, a prolonged reserpine treatment produces marked changes in D1 and D2 receptors increasing the proportion of high affinity state subpopulations and the total Bmax of D2 receptors. Also, dopamine function may be enhanced through an increment of the catalytic component of striatal adenylate cyclase.     

Electrophysiologic and biochemical studies on the putative Ca++ channel blocker MDL 12,330A in an endocrine cell

MDL 12,330A is a molecule structurally unrelated to other organic Ca++ channel ligands that may alter Ca++ channel function. Using whole cell patch clamp, [3H]PN200-110 binding and 45Ca++ uptake studies, we examined the effects of this compound on voltage-dependent Ca++ channels in rat anterior pituitary cells. At a concentration of 10(-5) M, MDL 12,330A showed little effect on outward K+ current, Na+ current or low-threshold Ca++ current in this cell line. At 10(-6) M, MDL 12,330A reversibly inhibited slow Ca++ current in a voltage-dependent manner. 45Ca++ uptake was also blocked by this compound at 10(-6) M, whereas [3H]PN200-110 binding was stimulated at concentrations of 10(-7) to 10(-6) M. The results are consistent with an interaction of MDL 12,330A with slow Ca++ channels at a site allosterically linked to the 1,4-dihydropyridine binding site.     

Effect of morphine on synaptosomal Ca++ uptake

The effect of morphine on the uptake of 45Ca++ was studied in synaptosomes from mouse brain using two procedures, centrifugation and filtration. The addition of morphine (1.7 x 10(-7) or 3.4 x 10(-7) M) reduced 45CA++ uptake by either technique, although the basal 45Ca++ uptake by the filtration method was approximately 7-fold higher than that by the centrifugation procedure. Similar effects were obtained after acute morphine treatment with 10 mg/kg s.c. Previous naloxone in vitro treatment (1.9 x 10(-8) M) or in vivo administration (2 mg/kg s.c.) reversed the morphine inhibition of the 45Ca++ uptake. On the other hand, after the animal was rendered tolerant and dependent by morphine pellet implantation, an enhancement of the synaptosomal 45Ca++ uptake was observed. It is concluded that changes in Ca++ fluxes in synaptosomes observed after acute and chronic morphine treatment may be involved with morphine pharmacological action related with analgesia, tolerance and physical dependence.     

Calcium transport abnormality in uremic rat brain synaptosomes.

Brain calcium is elevated in patients and laboratory animals with uremia. The significance of this finding is unclear. We evaluated calcium transport in brain of both normal and acutely uremic rats (blood urea nitrogen = 250 mg/dl) by performing studies in synaptosomes from rat brain cerebral cortex. Synaptosomes are vesicular presynaptic nerve endings from brain that contain mitochondria and are metabolically active. Two mechanisms of calcium transport were evaluated using radioactive 45Ca++ as a tracer. Both mechanisms were evaluated in the absence of exogenously administered parathyroid hormone (PTH). We first evaluated Na+-Ca++ exchange in vesicles that were loaded with NaCl in an external media containing 10 microM CaCl2. Both initial rates of calcium transport and equilibrium levels of calcium accumulation in synaptosomes prepared from uremic rats were significantly greater (P less than 0.005) than in normal. To assess calcium efflux, ATP-dependent calcium uptake (1 mM ATP) was studied in inverted plasma membrane vesicles loaded with KCl. In the uremic synaptosomes, a significant increase (P less than 0.005) in ATP-dependent calcium uptake was observed as compared with the normal. These studies show that (a) Calcium accumulation via the Na+-Ca++ exchanger is increased in synaptosomes prepared from uremic rat brain. (b) Calcium influx into inverted plasma membrane vesicles from uremic rats via the ATP-dependent calcium transport mechanism is increased when compared with normal. (c) The increased calcium accumulation in uremia by both Na+-Ca++ exchange and ATP-dependent calcium transport mechanism appears to be a result of increased synaptosomal membrane permeability to calcium. Both these abnormalities of calcium transport in uremia would tend to increase brain extracellular calcium in vivo. The defects observed in uremia do not appear to be readily reversible, and the relationship to PTH is presently unclear. These abnormalities may affect neurotransmission in the uremic state.     

Inositol 1,4,5-trisphosphate may regulate rat brain Cai++ by inhibiting membrane bound Na(+)-Ca++ exchanger.

The role of inositol 1,4,5-trisphosphate (1,4,5-IP3) in regulating cytosolic Ca++ by stimulating Ca++ release from intracellular organelles is well established. However, other modes of intracellular Ca++ regulation by 1,4,5-IP3 have not been determined. To determine if 1,4,5-IP3 may regulate cell cytosolic Ca++ by acting on plasma membrane bound Na(+)-Ca++ exchanger, we investigated Ca++ transport in synaptosomes using 45Ca++ as tracer. In the presence of either an inhibitor of voltage gated Na+ channels (tetrodotoxin) or the K+ ionophore (valinomycin), Ca++ uptake was significantly inhibited (P less than 0.05) by 1,4,5-IP3 in a concentration dependent manner, with half-maximal inhibition occurring at submicromolar concentrations between 10(-9) M and 10(-10) M 1,4,5-IP3. Similarly, Ca++ efflux by the exchanger was significantly inhibited 40% by 1,4,5-IP3. The inhibitory effect of 1,4,5-IP3 on the Na(+)-Ca++ exchanger was observed in the presence of Ca++ channel blockers, and in vesicles pretreated with caffeine to deplete the 1,4,5-IP3-sensitive stores of Ca++. These results suggest that during signal transduction in brain, 1,4,5-IP3 may increase cytosolic [Ca++] in part by inhibiting the Na(+)-Ca++ exchanger and thus, Ca++ efflux from cell.