Stimulation of renin secretion by catecholamines is dependent on adenylyl cyclases 5 and 6

The sympathetic nervous system stimulates renin release from juxtaglomerular cells via the β-adrenoreceptor-cAMP pathway. Recent in vitro studies have suggested that the calcium-inhibited adenylyl cyclases (ACs) 5 and 6 possess key roles in the control of renin exocytosis. To investigate the relative contribution of AC5 and AC6 to the regulation of renin release in vivo we performed experiments using AC5 and AC6 knockout mice. Male AC5(-/-) mice exhibited normal plasma renin concentrations, renal renin synthesis (mRNA and renin content), urinary volume, and systolic blood pressure. In male AC6(-/-) mice, plasma renin concentration (AC6(-/-): 732 ± 119; AC6 (+/+): 436 ± 78 ng of angiotensin I per hour*mL(-1); P<0.05), and renin synthesis were stimulated associated with an increased excretion of dilute urine (1.55-fold; P<0.05) and reduced blood pressure (-10.6 mm Hg; P<0.001). Stimulation of plasma renin concentration by a single injection of the β-adrenoreceptor agonist isoproterenol (10 mg/kg IP) was significantly attenuated in AC5(-/-) (male: -20%; female: -33%) compared with wild-type mice in vivo. The mitigation of the plasma renin concentration response to isoproterenol was even more pronounced in AC6(-/-) (male: -63%; female: -50% versus AC6(+/+)). Similarly, the effects of isoproterenol, prostaglandin E2, and pituitary adenylyl cyclase-activating polypeptide on renin release from isolated perfused kidneys were attenuated to a higher extent in AC6(-/-) (-51% to -98% versus AC6(+/+)) than in AC5(-/-) (-31% to 46% versus AC5(+/+)). In conclusion, both AC5 and AC6 are involved in the stimulation of renin secretion in vivo, and AC6 is the dominant isoforms in this process.     

Dopaminergic inhibition of anterior pituitary adenylate cyclase activity and prolactin release: the effects of perturbing calcium on catalytic adenylate cyclase activity

The dopaminergic inhibition of anterior pituitary adenylate cyclase activity, cAMP accumulation, and prolactin release was studied in the presence of the Ca2+ channel activator, maitotoxin. In isobutylmethylxanthine (IBMX)-treated cells, maitotoxin stimulated prolactin secretion within 30 s and cAMP accumulation within 1 min. Although dopamine reduced cAMP accumulation and prolactin release, the effectiveness of the catecholamine was reduced in the presence of maitotoxin. When hemipituitary glands were exposed for 10 min to 100 ng maitotoxin/ml, their membranes showed increased adenylate cyclase activity. The hypothesis that maitotoxin stimulates adenylate cyclase activity by increasing Ca2+ availability was supported by the observation that, at concentrations up to 100 microM, Ca2+-stimulated anterior pituitary adenylate cyclase activity. Although dopamine decreased basal and maitotoxin-stimulated pituitary cAMP accumulation, via changes in adenylate cyclase activity, the decrement in cyclic nucleotide production, but not prolactin release, can be ascribed to the effect of the catecholamine on the basal activities of these parameters. These data provide additional evidence that an increased Ca2+ flux is stimulating to cAMP generation and prolactin release, whereas dopamine is inhibitory to these processes.     

The role of calcium in the control of renin release

The effects of removing external calcium and inhibiting entry of calcium into the cell by treatment with D-600 on renin release from renal cortical slices of male Sprague-Dawley rats were examined. Baseline renin release, angiotensin II (AII)-induced inhibition, and isoproterenol-induced stimulation of renin release were studied. Removal of external calcium by chelation with 5 mM EGTA inhibited basal renin release while treatment with 1 mM EGTA stimulated basal renin release slightly. Incubation of slices with zero calcium medium containing 1 mM EGTA had no effect on isoproterenol-induced stimulation of renin release. In contrast, similar treatment reduced the inhibitory effect of AII from 58.7% of baseline to 85.3% (p less than 0.001). Similarly, blockage of calcium entry into cells with D-600 had no effect on isoproterenol-induced stimulation of renin release but abolished AII-induced inhibition. Replacement of sodium in the bathing medium with choline had no effect on baseline renin release or on AII-induced inhibition of renin release, ruling out the possibility that D-600 altered renin release via an effect on sodium influx. Taken together, the data strongly suggest that AII-induced inhibition of renin release is partially dependent on the presence of external calcium but that isoproterenol-induced stimulation of renin release is not.     

The effects of magnesium on calcium inhibition of parathyroid adenylate cyclase

cAMP has been shown to mediate the response of the parathyroid gland to a number of agonists and appears to take part in the regulation of this gland by divalent cations as well. We have studied the effects of the concentrations of free magnesium (Mg+2) and ionic calcium (Ca+2) on the kinetic properties of normal porcine parathyroid adenylate cyclase. In a previous study we obtained evidence for two calcium inhibition sites in this enzyme complex. In the present study we observed that the Mg+2 concentration influences the relative contribution of these sites to the overall calcium inhibition. At a high Mg+2 concentration (10 mM), the high affinity site contributes less than 50% of the total calcium inhibitable activity, whereas at a Mg+2 concentration in the low physiological range (0.5 mM), the high affinity site accounts for all the calcium inhibitable activity. Mg+2 was found to be a potent activator of porcine parathyroid adenylate cyclase, with a Ka of Mg of 0.8-2 mM. Ionic calcium at low concentrations (0.2-5 microM) acts as a competitive inhibitor with respect to Mg+2 activation. The calcium inhibition constant was estimated to be 2-3 microM. The Km for ATPMg-2 was 0.3 mM, which is similar to that found in other studies of adenylate cyclase activity in parathyroid tissue. The effects of Ca+2 on enzymatic activity with respect to the ATPMg-2 concentration showed noncompetitive inhibition. The calcium inhibition constant with respect to its effect on Vmax (KIv) was 3 microM; the calcium inhibition constant with respect to its effect on the binding of ATPMg-2 (KIs) was 10 microM. It is concluded from these results that the concentrations of intracellular Ca+2 reported to be present in parathyroid cells could inhibit adenylate cyclase activity. The mode of calcium inhibition that involves competition with magnesium would be particularly significant at low intracellular Mg+2 concentrations, and this phenomenon may account for the parathyroid secretory defect which is a characteristic feature of the magnesium-deficient state.     

Regulation of renin release by calcium and ammonium ions in normal man.

The effect of infusing calcium chloride, magnesium sulfate, sodium lactate, and ammonium chloride on renin secretion was compared to equimolar infusions of hypotonic and normal saline in sodium-deplete normal subjects. The infusion of 75 mEq of ammonium chloride for 60 min in 6 normal, sodium-deplete subjects suppressed plasma renin activity significantly (P less than 0.01) from 4.4 +/- 0.8 to 2.1 +/- 0.2 ng/ml/h, an effect comparable to that produced by normal saline. Sodium lactate (75 mEq sodium/hr) also significantly reduced renin levels at 20-30 min (P less than 0.01). The infusion of 1/3 normal saline (25 mEq sodium/h for 2 h) produced a significant reduction (P less than 0.01) in plasma renin activity (from control levels of 5.2 +/- 0.8 to 3.1 +/- 0.6 ng/ml/h at 90 min). On the other hand, comparable infusions of 50 mEq of magnesium sulfate over 2 h had no effect on renin release (4.6 +/- 0.8 to 4.6 +/- 0.9 ng/ml/h at 2 h), while the infusion of calcium chloride produced an intermediate reduction (5.2 +/- 1.2 to 3.7 +/- 0.8 ng/ml/h at 2 h (P less than 0.05). The observed effects of the hydrogen and calcium ions on suppressing renin release may be secondary to their known actions on renal sodium excretion. Since the infusions of calcium and hydrogen ions both result in an increased delivery of sodium to the distal segment of the nephron, the results may reflect the regulation of renin by the macula densa, a sensitive intrarenal sensor of renal tubular sodium.     

Effect of baroreceptor denervation on the inhibition of renin release by vasopressin

Previous studies have suggested that the inhibition of renin secretion by acute administration of vasopressin in conscious dogs results from a reflex reduction in renal nerve activity. In the present investigation, this hypothesis was tested by studying the effect of total baroreceptor denervation or selective low pressure baroreceptor denervation on the suppression of PRA by vasopressin in conscious, chronically prepared dogs. In eight sham-operated dogs, a 45-min infusion of vasopressin (2.0 ng/kg.min, iv) decreased PRA from 10.5 +/- 1.9 to 5.9 +/- 1.0 ng/ml.3 h (P less than 0.01). Mean arterial pressure did not change (110 +/- 10 to 107 +/- 7 mm Hg), but heart rate decreased from 84 +/- 9 to 69 +/- 8 beats/min (P less than 0.05). In contrast, vasopressin infusion failed to significantly decrease PRA in seven sinoaortic/cardiac denervated dogs (9.5 +/- 1.7 to 7.4 +/- 2.0 ng/ml.3 h), although decreases did occur in three of the dogs. Mean arterial pressure increased from 104 +/- 5 to 125 +/- 6 mm Hg (P less than 0.01), but heart rate did not change (112 +/- 4 to 107 +/- 5 beats/min). When renal perfusion pressure was maintained at the preinfusion level in three sinoaortic/cardiac denervated dogs, vasopressin infusion failed to decrease PRA (2.3 +/- 0.6 to 2.4 +/- 0.6 ng/ml.3 h). In six cardiac denervated dogs, vasopressin infusion decreased PRA from 5.3 to 0.9 to 3.1 +/- 0.7 ng/ml.3 h (P less than 0.01). Results obtained with two lower doses of vasopressin (0.5 and 1.0 ng/kg.min) were generally similar to the responses observed during infusion at 2.0 ng/kg.min. Angiotensin II (5.0 ng/kg.min) suppressed PRA in all groups of dogs. These experiments demonstrate that the inhibition of renin secretion by acute administration of vasopressin in conscious dogs is prevented by total baroreceptor denervation, but not by denervation of the low pressure baroreceptors alone. These results suggest that the suppression of renin release by vasopressin is a reflex response resulting from activation of the high pressure baroreceptors.     

Barium-induced exocytosis is due to internal calcium release and block of calcium efflux.

The concentration of cytosolic free Ca2+ ([Ca2+]i) and the release of tritiated norepinephrine ([3H]NE) were monitored during Ba2+ stimulation of sympathetic neurons cultured from chick embryos. Ba2+ (2.5 mM in Ca(2+)-free medium) caused a rise in [Ca2+]i in all regions (cell bodies, neurites, and growth cones) of sympathetic neurons and evoked [3H]NE release in the absence of other stimuli. The increase in [Ca2+]i and release of [3H]NE were sustained for up to 30 min in the presence of Ba2+. When Ba(2+)-stimulated cells were immediately washed in Ca(2+)-free Ba(2+)-free EGTA solution, both the elevated [Ca2+]i and [3H]NE release returned to basal levels, with similar, fast, time courses. Ba2+ also blocked Ca2+ efflux from neurons loaded with 45Ca. We conclude from the parallel effects of Ba2+ on [Ca2+]i and [3H]NE release that Ba2+ stimulates exocytosis by a Ca(2+)-dependent mechanism. The Ba(2+)-induced rise in [Ca2+]i is a result of two separate actions: (i) the release of Ca2+ from intracellular sites and (ii) an effective block of Ca2+ extrusion. The ability of Ba2+ to release Ca2+ in growth cones that are insensitive to caffeine suggests that Ba2+ may displace Ca2+ from binding sites other than endoplasmic reticulum.     

Pertussis toxin attenuates angiotensin II-induced vasoconstriction and inhibition of renin release

Angiotensin II can elicit cellular responses by 2 different receptor-dependent mechanisms: increase in intracellular calcium or inhibition of adenylate cyclase activity. The well-known inhibition of renin release from granulated cells of the kidney is thought to be mediated by an increase in intracellular calcium. However, the participation of the other possible pathway, i.e. inhibition of adenylate cyclase, has not been excluded. We studied this question by using the toxin from Bordetella pertussis, which inactivates the inhibitory coupling units Ni and thus permits to identify hormonal actions mediated through inhibition of adenylate cyclase. In isolated perfused kidneys from rats pretreated with pertussis toxin (2 micrograms/100 g i.v., single injection) the inhibition of renin release by angiotensin II (10(-11) to 10(-8) M) was significantly attenuated. In parallel, the vasoconstrictor response to angiotensin II was also diminished in these rat kidneys. The effect of pertussis toxin was apparent 3, 5 and 10 days after treatment, with a maximal effect at the fifth day. These data suggest that angiotensin II may exert the inhibitory effect on renin release in part through inhibition of adenylate cyclase in granulated cells of the kidney.     

Regulation of growth hormone release in common carp pituitary cells by pituitary adenylate cyclase-activating polypeptide: signal transduction involves cAMP- and calcium-dependent mechanisms

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a member of the glucagon/secretin peptide family and its molecular structure is highly conserved among vertebrates. In this study, the role of PACAP in regulating growth hormone (GH) secretion in fish was examined in vitro using common carp pituitary cells under column perifusion. A dose-dependent increase in GH release was observed after exposing pituitary cells to increasing doses of ovine PACAP38 (oPACAP38) and PACAP27 (oPACAP27), but not vasoactive intestinal polypeptide (VIP). A lack of GH response to VIP stimulation is consistent with the pharmacological properties of PAC-1 receptors, suggesting that this receptor subtype may be involved in PACAP-induced GH secretion in carp species. Although the maximal GH responses induced by oPACAP38 and oPACAP27 were similar, the minimal effective dose and ED50 value for oPACAP38 were significantly lower than that for oPACAP27. These results may indicate that common carp PAC-1 receptors are more sensitive to stimulation by oPACAP38 than by oPACAP27. In parallel studies, oPACAP38 and oPACAP27 were also effective in increasing cAMP release, cellular cAMP content, total cAMP production, and intracellular Ca(2+) ([Ca(2+)](i)) levels in common carp pituitary cells. Besides, the rise in [Ca(2+)](i) induced by oPACAP38 was blocked by removing extracellular Ca(2+) ([Ca(2+)](e)) or by treatment with nifedipine, an inhibitor of voltage-sensitive Ca(2+) channels (VSCC). The dose dependence of PACAP-stimulated GH release in common carp pituitary cells was mimicked by activating adenylate cyclase using forskolin, inhibiting cAMP degradation using IBMX, increasing functional levels of intracellular cAMP using CPT-cAMP, or inducing [Ca(2+)](e) entry using the Ca(2+) ionophore A23187. In contrast, the GH-releasing effect of oPACAP38 was suppressed by treatment with the adenylate cyclase inhibitor MDL12330A, protein kinase A inhibitor H89, and VSCC blocker nifedipine, or by perifusion with a Ca(2+)-free culture medium. These results, as a whole, suggest that PACAP functions as a GH-releasing factor in common carp by activating pituitary receptors resembling mammalian PAC-1 receptors. Apparently, the GH-releasing action of PACAP is mediated through the adenylate cyclase/cAMP/protein kinase A pathway and [Ca(2+)](e) influx through VSCC.     

Mechanism of renin inhibition by beta adrenergic blocking agents. Effects of dl-, d-, l-propranolol and pindolol on renin release

In order to reveal the mechanism of renin inhibition by beta adrenergic blocking agents, the effects of dl-, d-, l-propranolol and pindolol on renin release were studied. This was done by injecting them intraperitoneally or by using an in vitro system of rat kidney slices. In the in vivo study, dl-, d-, and l-propranolol inhibited plasma renin activity and renal renin content significantly in normal rats. Furthermore, in the in vitro study, the basal levels of renin in the media and that in the kidney were significantly inhibited by these agents. Pindolol also inhibited renin release, but its effects were significantly less than those of other agents. The finding that d-propranolol which has little beta adrenergic blocking action inhibited renin release, and that the effects of pindolol which displays strong beta adrenergic blocking action but little membrane stabilizing action, were less than those of other agents, may suggest that the inhibitory effects of beta adrenergic blocking agents on renin release are dependent mainly on the membrane stabilizing action rather than the beta adrenergic blocking action.     

The effect of theophylline and cyclic adenosine 3',5'-monophosphate on renin release by afferent arterioles

The effects of theophylline and cyclic adenosine 3',5'-monophosphate (cAMP) on renin release by afferent arterioles were studied. Rabbit afferent arterioles (seven to 10), obtained by a microdissection technique, were incubated for three consecutive 20 min periods in 100 microliters of Medium 199 with 0.1% bovine serum albumin (BSA). Afferent arterioles exposed to theophylline, 1 x 10(-4) mol/l, produced a greater than 100% increase in renin release (0.18 +/- 0.04 to 0.42 +/- 0.05 ng angiotensin l/h per arteriole per h incubation). The renin release stimulated by theophylline was completely abolished by indomethacin and meclofenamate. The stimulation of renin secretion was also blocked when extracellular calcium concentration was decreased to 10(-7) mol/l and the arterioles were permeabilized with calcium ionophore. Dibutyryl cAMP (db-cAMP), 1 x 10(-4) mol/l, and forskolin, 1 mumol/l or 100 mumol/l, failed to stimulate renin release by afferent arterioles. The results of this study therefore suggest that theophylline-stimulated renin release may be mediated through a prostaglandin pathway. The stimulation of renin secretion is also dependent on the extracellular concentration of calcium.     

Role of extra- and intracellular calcium and calmodulin in renin release from rat kidney

Renin release from the juxtaglomerular cell appears to be inversely related to calcium concentration. We studied the role of Ca+2 to confirm recent findings and to further explore the role of intracellular calcium as well as the calcium-calmodulin system in renin release. A rat renal cortical slice preparation was used. Isoproterenol (10(-6) M) caused significant stimulation of renin release, whereas angiotensin (AII; 5 X 10(-5) M) suppressed basal as well as isoproterenol-stimulated renin release. Removal of calcium from the buffer reversed AII suppression of renin release. Nifedipine (10(-5) M), a specific calcium channel blocker, induced a marked increase in basal renin release. TMB-8, an inhibitor of intracellular calcium release, also caused a dose-related increase in basal renin release. The calmodulin antagonists trifluoperazine and calmidazolium both caused significant dose-related increases; however, calmidazolium was a more potent stimulator. Low extracellular calcium or nifedipine concentrations did not alter isoproterenol-induced renin release. Isoproterenol further stimulated renin release in the presence of trifluoperazine and calmidazolium. These results suggest that acute beta-adrenergic stimulation of renin in independent of changes in levels of extracellular and intracellular calcium and calmodulin. These studies provide further evidence that unlike most other secretory systems, the reduction of intracellular calcium or inhibition of the calcium-calmodulin system in the juxtaglomerular cells of the kidney acts as a secretogogue.     

The mechanism of the control of renin release by beta-adrenergic receptors

Recent reports suggest that prostaglandins play an important role in the beta-adrenergic receptor mechanism of renin release. However, the site of the action of prostaglandins has not yet been clarified. Superfusion of rabbit renal cortical slices was used to evaluate the beta-adrenergic receptor mechanism of renin release. Renin release was stimulated by isoproterenol, prostaglandin E2, and dibutyryl cyclic AMP. Renin release stimulated by isoproterenol was inhibited by propranolol, whereas renin release stimulated by prostaglandin E2 was not inhibited by propranolol. Isoproterenol stimulated prostaglandin E2 release as well as renin release, and indomethacin inhibited these effects of isoproterenol. Propranolol inhibited prostaglandin E2 release stimulated by isoproterenol. On the other hand, indomethacin did not affect renin release stimulated by prostaglandin E2 release. Dibutyryl cyclic AMP did not stimulate prostaglandin E2 release. Indomethacin did not affect renin release stimulated by dibutyryl cyclic AMP, however, it suppressed prostaglandin E2 release during the superfusion with dibutyryl cyclic AMP. Finally, isoproterenol and prostaglandin E2 stimulated cyclic AMP release. These data suggest that prostaglandins play an important role in the beta-adrenergic receptor mechanism of renin release and the site of the action of prostaglandins is between the beta-adrenergic receptor and cyclic AMP.     

Modification of intracellular calcium and plasma renin by dietary calcium in men

A double blind, placebo-controlled, parallel study was conducted on the effect of a high daily oral calcium supplementation of 1 g elemental calcium, given twice daily for 16 weeks in normal male subjects, on plasma renin, aldosterone, kallikrein, cGMP, cAMP, and calciotropic hormones, intracellular calcium concentrations, and plasma total and ionized calcium. After a 1-month run-in period on a limited use of dairy products, the subjects (n = 32) were allocated to a placebo or a calcium group. Placebo or 1 g elemental calcium was administered twice daily in the morning and evening for 16 weeks. All subjects were investigated at baseline and after 1, 2, 4, 8, and 16 weeks of placebo or calcium administration.

A decreased intraerythrocyte and intraplatelet Ca²⁺ concentration was observed in the calcium-treated subjects. Compared with the placebo group, an increase in the plasma renin activity (PRA) in the calcium group was observed after 4, 8, and 16 weeks of oral calcium administration. However, plasma aldosterone and urinary excretion of aldosterone, kallikrein, cGMP, and cAMP were not changed during calcium administration. Oral calcium supplementation in these men was also accompanied by a reduction in the plasma concentration of intact parathyroid hormone and 1,25-dihydroxyvitamin D₃, and an increase in 24-h urinary calcium excretion, but no change in the plasma total Ca²⁺ concentration, serum ionized Ca²⁺ level, and plasma phosphate or 25-hydroxyvitamin D₃.Our data show that the increase in PRA observed in men during oral calcium supplementation is accompanied by a reduction in the intracellular free and total Ca²⁺ concentration in platelets and erythrocytes and by a decrease in the plasma concentration of intact parathormone and 1,25-dihydroxyvitamin D₃.     

Transfected cGMP-dependent protein kinase suppresses calcium transients by inhibition of inositol 1,4,5-trisphosphate production.

cGMP is a key regulatory molecule in visual transduction, integration of neuronal response to excitatory neurotransmitters, relaxation of smooth muscle, intestinal secretion of water and salt, and reabsorption of sodium and water in the distal tubules of the nephron. Some of these cellular functions are associated with the activation of cGMP kinase and a decrease in cytosolic calcium levels ([Ca2+]i). The mechanism by which cGMP kinase lowers [Ca2+]i is controversial. We have used CHO cells stably transfected with cGMP kinase to test several of the proposed [Ca2+]i-lowering mechanisms. Thrombin induces a calcium transient in wild-type and cGMP kinase-expressing CHO cells by releasing calcium from intracellular stores. Preincubation of wild-type cells with 8-bromo-cGMP had no effect on the calcium transient, whereas 8-bromo-cGMP prevented the thrombin-stimulated calcium transient in cGMP kinase-expressing CHO cells. In both cell types 8-bromo-cGMP had no effect on [Ca2+]i transients induced by replacing extracellular sodium by tetramethylammonium, ruling out an effect of cGMP kinase on Ca(2+)-ATPases. However, cGMP kinase activation effectively suppressed thrombin-induced stimulation of inositol 1,4,5-trisphosphate production. These results show that cGMP kinase lowers [Ca2+]i by interfering with the inositol 1,4,5-trisphosphate synthesis.     

Enhanced spontaneous calcium efflux and decrease of calcium-dependent calcium release from the isolated perfused heart of spontaneously hypertensive rats.

OBJECTIVE: The aim of this study was to clarify the further details of calcium handling in hypertension. DESIGN: By preserving the physiological environment of cell membrane, whole hearts were used for comparison of calcium flux between spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats. METHODS: Hearts from SHR and WKY rats were perfused with Krebs-Henseleit solution under constant flow and the effluent collected. RESULTS: After labelling of the heart with 45Ca2+ (100 mumol/l), 45Ca2+ binding was found to be saturated, and washing with calcium-free perfusion solution showed two exponential curves for calcium dissociation, indicating a fast (alpha-) and slow (beta-) phase. The half-lives of the beta-phase for both 4- and 8-week-old SHR were significantly shorter than those for age-matched WKY. Also in this phase, infusion of non-radioactive Ca2+ caused a transient dose-dependent release of 45Ca2+. A significant reduction in the amount of 45Ca2+ release induced by 2 mmol/l Ca2+ was observed in both 4- and 8-week-old SHR compared with age-matched WKY rats. Infusion of lanthanum, caffeine, ionomycin (calcium ionophore) and treatment of the hearts with ethyleneglycol-bis-(beta-aminoethylether)-N,N,N,',N'-tetraac etic acid did not alter 45Ca2+ release by non-radioactive Ca2+. From these observations, 45Ca2+ is presumably released from the intracellular calcium pool, and not from extracellular binding sites or sarcoplasmic reticulum. CONCLUSIONS: These findings suggest that an abnormal calcium-handling defect (enhanced calcium efflux and reduction of membrane-bound Ca2+) exists under physiological conditions before and after the onset of hypertension, and that this may be a primary characteristic of SHR.     

The inhibition of adenylate cyclase in equine platelets by collagen and by platelet-activating factor

Equine platelet aggregation was stimulated by collagen fibres or platelet-activating factor. The action of both ligands was blocked by forskolin or prostaglandin E₁ agents which are known to activate adenylate cyclase. Equine platelet membranes were found to contain adenylate cyclase activity which was inhibited in dose-dependent fashion by both collagen and platelet-activating factor. Platelet-activating factor-induced inhibition was antagonised by WEB2086.     

The inhibition of adenylate cyclase in equine platelets by collagen and by platelet-activating factor

Equine platelet aggregation was stimulated by collagen fibres or platelet-activating factor. The action of both ligands was blocked by forskolin or prostaglandin E(1) agents which are known to activate adenylate cyclase. Equine platelet membranes were found to contain adenylate cyclase activity which was inhibited in dose-dependent fashion by both collagen and platelet-activating factor. Platelet-activating factor-induced inhibition was antagonised by WEB2086.