Altered myocardial response to ouabain in diabetic rats: mechanics and electrophysiology

Diabetes induced by streptozotocin in rats is associated with changes in the mechanical function of isolated ventricular papillary muscle. Relaxation is slowed and shortening velocity is depressed. The effects of ouabain (10(-7) to 2 x 10(-4)M) and changes in extracellular calcium ([Ca2+]0 = 0.6 to 12 mM) on the mechanical and electrical properties of normal and diabetic papillary muscles were studied. High doses of ouabain caused a rise in resting tension and a fall in developed tension in both diabetic and control muscles. However, these changes were strikingly greater in diabetic muscles which developed partial contractures at 10(-4)M. The altered response to ouabain was observed in chronically (5 to 7 weeks or 3 months) but not acutely (less than 1 week) diabetic animals. Similarly, the response to ouabain was restored to normal after chronic (5 weeks) therapy with insulin but not after acute (4 days) therapy. In both normal and diabetic muscles, the mechanical effects of increasing [Ca2+]0 from 2.4 to 12.0 mM were qualitatively similar to those seen with ouabain at 10(-5) to 10(-4)M. Electrophysiologic studies showed that under control conditions action potentials of diabetic muscles were significantly longer than those of normal muscles. Treatment with progressively higher concentrations of ouabain (10(-7) to 10(-4)M) and [Ca2+]0 (2.4 to 12.0 mM) caused shortening of both normal and diabetic action potentials, but the effects of these interventions were much greater in the diabetics. These results suggest that the response of diabetic muscles to ouabain is markedly different from normal and that this altered response may be due to impaired regulation of intracellular Ca2+ levels in diabetic myocardium.     

Depression of contractions of rabbit aorta and guinea pig vena cava by mesudipine and other slow channel blockers

The effects of several drugs having Ca++-antagonistic and vasodilating properties were compared in arterial and venous smooth muscles. Developed force (phasic component) was recorded from isolated rings (about 2 mm wide) of blood vessel wall taken from rabbit aorta or guinea pig inferior vena cava. The vascular smooth muscle (VSM) was stimulated to contract for a sustained period by elevating the extracellular K+ concentration ([K]o) to 100 mM or by exposure to norepinephrine (NE). All drugs depressed the K+-induced contractures in a dose-dependent manner between 10(-9) and 10(-5) M. The order of potency in aorta was: mesudipine = verapamil greater than diltiazem greater than nifedipine. Of the three drugs studied in vena cava, the order of potency was: mesudipine greater than verapamil greater than bepridil. It is concluded that, in both preparations of arterial and venous VSM, mesudipine is the most potent inhibitor of K+-induced contractions. Aortic contractions to 10(-7) M NE were depressed at concentrations of Ca++ antagonists 2 or 3 orders of magnitude less than those needed to depress contractions to 10(-5) M NE. The NE-induced contractions were depressed by the drugs to about the same extent as the K+-induced contractions when 10(-7) M NE was used, but were depressed to a much smaller extent when 10(-5) M NE was used. This may reflect the involvement of intracellular Ca++ storage sites in contractions induced by high NE concentrations. It is likely that these drugs depress and block Ca++ influx through the cell membrane.     

Maximal Ca2+-activated force elicited by tetanization of ferret papillary muscle and whole heart: mechanism and characteristics of steady contractile activation in intact myocardium

Rapid (8-12 Hz) stimulation of intact heart muscle treated with ryanodine results in steady contractile activation known as tetanus, the amplitude of which can be graded by changing extracellular Ca2+ concentration ([Ca2+]o). The mechanism of the sustained force generation was explored in ferret papillary muscles by measuring membrane potential and by determining the responsiveness of force and intracellular free Ca2+ concentration ([Ca2+]i, estimated with aequorin) to dihydropyridine Ca channel ligands. Membrane potential during tetani ranged from -25 to -60 mV, suggesting that fast or slow Ca channels, or Na-Ca exchange, might be mediating Ca2+ entry. Dihydropyridine effects indicated that slow Ca channels play a predominant role: The agonist Bay K 8644 (0.3-1 microM) increased force and aequorin luminescence, whereas the antagonist nitrendipine (1-30 microM) abolished the tetanus. Under conditions analogous to those in the papillary muscle experiments, tetani were produced in whole Langendorff-perfused ferret hearts following exposure to ryanodine. Contraction saturated as a function of [Ca2+]o in both papillary muscles and whole hearts; i.e., as [Ca2+]o was increased above 10 mM, no further increase in force or pressure generation occurred. In contrast, aequorin luminescence measured in the papillary muscles showed no such saturation. Thus, maximal Ca2+-activated force (or pressure) was achieved during tetani at [Ca2+]o greater than or equal to 10 mM. Calculations of wall stress during tetani in whole heart (15 mM [Ca2+]o) agree well with direct measurements of maximal tension in papillary muscles (5.84 g/mm2 vs. 6.41 g/mm2, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)     

Hyperreactivity and 45Ca movements in sensitized guinea-pig tracheal muscle.

Responses to KCl and histamine and 45Ca movements were studied in trachea from normal and actively sensitized guinea-pigs. Sensitized tracheas were hyperresponsive and hypersensitive to KCl and histamine. 45Ca uptake experiments show that sensitized tracheal muscle behaves as normal except that the uptake of 45Ca in low concentration (0.03 mM) Ca2+ solution was higher and the number of binding sites for the high affinity component of 45Ca uptake (as estimated by Scatchard-coordinate plot) was augmented. Additionally, in sensitized tracheal muscle, incubation in low (0.03 mM) Ca2+ solution followed by La3+ wash-out resulted in a greater amount of residual 45Ca than in normal tissues. KCl, but not histamine, increased the La3+ resistant 45Ca content. This increase was greater in sensitized than in normal trachea. This demonstrates the existence of hyperreactivity and altered 45Ca movements in sensitized trachealis muscle.     

In vitro caffeine induced aortic smooth muscle reactivity in rat

The effects of caffeine on aortic smooth muscle contractility during hypertension were studied in SHR and WKY control rats. To compare the effects of Mg++ on vascular reactivity induced by caffeine 1.2 mM MgCl2 was either included or omitted from the Krebs solution bathing the aortic tissue. The role of alpha-adrenergic receptors and verapamil-sensitive Ca++ channels in eliciting caffeine induced contraction in aortic tissues was also examined in Sprague Dawley rats. We report that the aortic smooth muscle from SHR animal was less responsive than WKY aortic smooth muscle to 10 and 20 mM concentrations of caffeine. Caffeine induced a relaxation of aortic smooth muscle contracted with 60 mM KCl or 10(-7) M NE. However, the relaxation response was slower in SHR as compared to WKY rats. To assess the involvement of alpha-adrenergic receptors in caffeine induced aortic contractility alpha 1- and alpha 2-receptors were blocked with 10(-7) M prazosin and 10(-7) M yohimbine respectively. The caffeine induced aortic contractility did not seem to involve alpha-adrenergic receptors. A blockade of verapamil sensitive Ca++ channels with 10(-7) M verapamil failed to inhibit caffeine induced aortic contractility. These results indicate that caffeine involves release of Ca++ in vascular muscle, however, Ca++ is released from a site other than the one controlled by alpha-adrenergic receptors. Also, the Ca++ channels involved are other than the Verapamil sensitive Ca++ channels. Yet it is clear that if the aortic contractility is due to Ca++ release alone, then caffeine is a potent agent for Ca++ release in the aortic smooth muscle of rat. Additionally, the caffeine-sensitive mechanism for aortic smooth muscle contraction is impaired during hypertension.     

Calcium flux and binding in the aortic smooth muscle from the spontaneously hypertensive rat.

There was no significant difference in the tissue calcium content between spontaneously hypertensive rat (SHR) and normotensive rat aortae. 45Ca uptake was significantly greater, whereas 45Ca influx was significantly less in SHR than in control aortae. There was no difference in 45Ca efflux in SHR and control tissues. 45Ca influx in both SHR and control tissues was increased by potassium, not increased by norepinephrine and inhibited by Co++, Mn++ and La+++ (all 0.5 mM). Both SHR and control aortae were equally inhibited by Mn++ and Co++ (0.1 mM). There was no difference in the 45Ca uptake by microsome fraction from both SHR and control tissue. These data indicate some differences in the translocation of Ca++ at the cellular level in SHR and control aortae.     

Mechanism of Ca++ antagonist-induced vasodilation. Intracellular actions

We studied the effects of Ca++ antagonists on intact and skinned muscles of rabbit mesenteric artery. Intact muscle contractions were inhibited by 10(-6) M diltiazem, whereas greater levels were required to abolish contractions in skinned muscle fibers. In contrast, nisoldipine had no effect on skinned muscle contractions, although it inhibited, almost completely, the contraction of intact muscle at concentrations below 10(-6) M. In the presence of EGTA, norepinephrine-induced contractions result from a release of Ca++ from an intracellular store. Diltiazem inhibited these contractions at concentrations between 10(-6) and 10(-4) M. Higher doses were required in studies with skinned muscle preparations. Unlike diltiazem, nisoldipine only partially inhibited the Ca++-free norepinephrine-induced contractions in the range of 10(-7) to 10(-5) M. From these results, we assumed that at low concentrations (below 10(-6) M), diltiazem induced relaxation by blocking Ca++ influx, whereas at relatively high concentrations (above 10(-6) M), an inhibition of Ca++ release from an intracellular store also occurred. A similar conclusion was reached regarding the mechanism whereby nisoldipine inhibits force developments.     

Role of Na+ and Ca++ in guinea pig trachealis contraction induced by cooling

The role of extracellular Na+ and Ca++ in cooling-induced contraction (CIC) was studied in guinea pig trachealis muscle. Tracheal preparations were tested in Krebs-Ringer bicarbonate (KRB) solution and then in either: (1) a low Na+ sucrose KRB, (2) KRB plus amiloride, a sodium channel blocker, or (3) KRB plus nifedipine, a calcium channel blocker. When the trachealis muscle was cooled from 37 to 20 degrees C in KRB, the value of the isometric tension increased on average by 25% of the maximal tension induced by acetylcholine; however, when the tissue was tested in the low Na+ sucrose KRB or the KRB containing amiloride, CIC was totally prevented. In the presence of nifedipine the mean value of CIC was only 10% of the acetylcholine-induced contraction, which was also observed not to be affected by extracellular Na+ or Ca++; whereas the KCl-induced contraction was affected by Ca++ but not by Na+. The results suggest that rapid cooling of the smooth muscle cells causes an increase in sodium conductance and a decrease in the activity of the Na+-K+ pump, resulting in membrane depolarization. During depolarization a significant quantity of Ca++ enters the cell, contributing to the magnitude of CIC.     

Trans-sarcolemmal Ca2+ movements associated with contraction of the rabbit right ventricular wall

The purpose of this study was to examine the movements of Ca2+ into the myocardium from the extracellular space during the course of muscle contraction. Equilibration of the rabbit right ventricular wall with perfusate containing 45Ca was measured by collecting effluent droplets over time. This protocol was carried out in a quiescent muscle and then repeated 15-20 minutes later in an identical fashion except that halfway through the collection period the muscle was stimulated to contract. We were thus able to quantitate the contraction-dependent changes in 45Ca movements. In control experiments using [58Co]EDTA as an extracellular space marker, we observed that contraction altered the volume of this space. This alteration in extracellular space was relatively small, and the quantitation of 45Ca movements was corrected for this change. The addition of 1 microM Bay K 8644 to the perfusate stimulated tension and augmented Ca2+ depletion from the extracellular space. The addition of 15 microM nifedipine to the perfusate significantly reduced both tension development and Ca2+ depletion from the extracellular space of the muscle. Net contraction-dependent movement of Ca2+ from the extracellular space into the myocardium under control conditions at 1 mM [Ca2+] was 10-14 mumol Ca2+/kg tissue wet wt/beat. This value indicates either a large contribution of Ca2+-induced Ca2+ release from the sarcoplasmic reticulum and/or significant contribution of sarcolemmal bound Ca2+ to excitation-contraction coupling in the rabbit ventricle.     

Effects of neomycin on 45Ca binding and distribution in canine arteries.

The effects of neomycin (7.0 mM) on 45Ca movements and distribution were investigated in canine aortae and in canine carotid and terminal mesenteric arteries. Uptake of 45Ca was measured in calcium-free solution; the 45Ca tissue spaces in the carotid and terminal mesenteric arteries were 2--4 times greater than those observed in the aorta. Exposure of the aortae and the terminal mesenteric arteries to 1.5 mM Ca++ during the washout elicited large increases in 45Ca efflux in both preparations (increase in terminal mesenteric greater than aorta). Moreover, in all three arterial preparations, neomycin reduced 45Ca uptake and induced a sustained increase in 45Ca efflux (effects on terminal mesenteric larger than or equal to carotid greater than aorta). The terminal mesenteric and carotid arteries may accumulate and bind 45Ca at superficial membrane sites (readily exchangeable 45Ca) to a greater degree than does the aorta. If Ca++ located at these membrane sites contributes directly to the maintenance of mechanical responsiveness, then agents which alter membrane binding of Ca++ (e.g. neomycin) may exert a stronger action on these highly reactive vessels. Thus, contractile responsiveness in peripheral arteries may depend upon depots of superficially bound Ca++ to a greater degree than in the more centrally located aorta.     

The inhibitory action of lidocaine in anaphylaxis.

The action of lidocaine, a local anesthetic, was investigated during anaphylaxis in guinea pigs after passive sensitization in vitro of lung tissue and trachealis muscle. Pretreatment of the trachealis muscle with 8.54 mM lidocaine resulted in the total inhibition of anaphylactic isometric tension. Full reversal of anaphylactic-induced contractures was rapidly achieved with concentrations of 4.27 mM lidocaine. Release of histamine from both lung tissue and trachealis muscle was inhibited by 73 to 82 per cent, respectively, over concentration ranges of 2.13 to 8.54 mM lidocaine. A bimodal effect on sensitized tissues was noted, with lidocaine causing a slight release of histamine in the trachealis muscle of 1.6 per cent at a concentration of 8.54 mM. Lidocaine did not impair the initial passive sensitization process, nor did it appear to elute antibody once it was cell bound. The dual inhibitory effect on mast-cell release of mediators and on muscle contraction by lidocaine may be related in part to common processes involving the binding or flux of calcium.     

Growth hormone release: interaction of increased extracellular calcium and somatostatin

These studies were designed to examine the effects of extracellular calcium ion (Ca++) concentration upon basal and dibutyryl (db) cAMP or potassium ion (K+)-stimulated release of growth hormone (GH) and to determine whether increased extracellular Ca++ can overcome somatostatin (SRIF)-inhibited release of stored rGH in parallel with its reported effect upon SRIF inhibition of stimulated insulin and glucagon release. Experiments were performed in vitro using prelabeled rat pituitary fragments in a perifusion, specific immunoprecipitation system designed to limit observations to release of stored hormone from viable cells. Increased (up to 5.4 mM) extracellular Ca++ inhibits basal and dbcAMP-stimulated release of stored, prelabeled [3H]rGH in parallel with the effects of SRIF: post-inhibition rebound, dose responsivity, and differential effect upon early and late dbcAMP-stimulated release of stored [3H]rGH. Increased (21 mM) extracellular K+ interferes with both Ca++- and SRIF-inhibited early dbcAMP-stimulated release of stored [3H]rGH. The combination of increased extracellular Ca++ and SRIF inhibits basal release of stored [3H]rGH more than either agent alone and during dbcAMP stimulation, rebound release of stored [3H]rGH follows withdrawal of either inhibitor despite continuation of the other. This rebound release is enhanced when both inhibitors are withdrawn simultaneously. Conclusions: (a) the inhibition of stored rGH release induced by increased extracellular Ca++ and SRIF occurs through at least partially independent mechanisms, and (b) increased extracellular Ca++ does not reverse SRIF inhibition of stimulated rGH release from prelabeled intracellular storage, in contrast with observations in the pancreatic islet.     

Hepatotoxic bile acids increase cytosolic Ca++ activity of isolated rat hepatocytes

Effects of bile acids on cystolic Ca++ activity and cell viability of isolated rat hepatocytes were studied to test the hypothesis that bile acids may produce hepatotoxicity by increasing cystolic Ca++ activity. Changes in cystolic Ca++ activity were calculated from time-dependent changes in fluorescence of quin-2 loaded hepatocytes. Release of lactate dehydrogenase and changes in propodium iodide fluorescence were used to assess cell viability. Bile acids studied were unconjugated and taurine-conjugated cholate, chenodeoxycholate (and taurochenodeoxycholate), deoxycholate (and taurodeoxycholate) and lithocholate (and taurolithocholate). With the exception of cholate and taurocholate, bile acids increased cystolic Ca++ activity within 10 to 30 sec in a concentration-dependent fashion (0.05 to 1.0 mM) and in the order lithocholate = taurolithocholate greater than chenodeoxycholate = taurochenodeoxycholate = deoxycholate = taurodeoxycholate. The initial increase in cystolic Ca++ activity by bile acids was not due to cell damage, since bile acid-induced decreases in cell viability were not significant until 2 to 3 min. At higher concentrations of unconjugated bile acid, there was a secondary increase in quin-2 fluorescence corresponding temporally to the increase in propodium iodide fluorescence, indicating cell damage after the initial increase in cystolic Ca++ activity. The ability of conjugated and unconjugated bile acids to increase cystolic Ca++ activity was abolished and decreased (60 to 90%), respectively, in the absence of extracellular Ca++, indicating that extracellular Ca++ is the major source of the bile acid-induced increase in cystolic Ca++ activity.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibition of parathyroid hormone release from dispersed bovine cells by ouabain

Ouabain, at concentrations of 3 X 10(-4)-10(-3) M, inhibited secretion of PTH approximately 50% in a freshly dispersed bovine parathyroid cell preparation. This inhibition was found with both unstimulated and secretagogue-stimulated PTH release. Reductions in PTH secretion were found at all concentrations of Ca++ tested between 0.3 mM and 2.0 mM and in the presence of the divalent cation chelators EDTA and EGTA, indicating that extracellular Ca++ is not an absolute requirement for the inhibition. The ouabain inhibition did not appear to be mediated through changes in either adenylate cyclase activity or total cellular cAMP, implying a locus distal to the generation of this cyclic nucleotide. The data suggest that transmembrane potential and/or distribution of monovalent cations across the plasma membrane is important in the maintenance of PTH secretion. The mechanisms involved in this control do not appear to involve extracellular Ca++ directly.     

Dependency of acetylcholine-induced insulin release on Ca++ uptake by rat pancreatic islets

The dependency of acetylcholine-induced insulin release on Ca++ uptake from extracellular fluid has been investigated in isolated rat pancreatic islets. Islets were maintained for 46 h in tissue culture, and 45Ca++ uptake and insulin release were measured over 5 min on the same batches of islets. Acetylcholine (10(-10)--10(-6) M) at 2.8 mM glucose stimulated insulin release and Ca++ uptake. Both effects were inhibited by atropine. When acetylcholine-stimulated Ca++ uptake was inhibited by verapamil, a blocker of the voltage-dependent Ca++ channel, acetylcholine-induced insulin release was abolished. In a previous report, verapamil did not inhibit glucose-stimulated insulin release over 5 min despite inhibition of the accompanying Ca++ uptake. Glucose (16.7 mM) stimulated insulin release to a similar extent as acetylcholine. Acetylcholine further enhanced glucose-stimulated insulin release without changing glucose-stimulated Ca++ uptake. Acetylcholine did not significantly alter cAMP levels in the islets. It is concluded that acetylcholine, in contrast to glucose, appears to stimulate the early phase of insulin release by enhancing Ca++ uptake from extracellular fluid. An additional effect of acetylcholine in the presence of high glucose may be operative.     

Divalent cations modulate PTH-dependent 3',5'-cyclic adenosine monophosphate production in renal proximal tubular cells.

The kidney and parathyroid gland play key roles in calcium (Ca++) homeostasis. Recent data suggest that the kidney, in addition to being a primary target for PTH, also recognizes changes in the concentration of extracellular Ca++, thereby modulating hormone-dependent cAMP production, 1,25-dihydroxyvitamin D synthesis, and renin secretion. In this study, we examined: 1) the effects of varying concentration of divalent cations on PTH-dependent cAMP production in renal proximal tubular cells; and 2) the mechanisms by which extracellular Ca++ exerts its inhibitory effects on cAMP production. Single cell suspensions composed of 80-90% proximal tubular cells were prepared from cortical homogenates by collagenase digestion and sieving. In the presence of 1 mM isobutylmethylxanthine, cAMP content was measured by RIA in 5-15 min incubations and showed a 5- to 6-fold increase in response to PTH (10(-11) -10(-6) M). Increasing extracellular Ca++ and magnesium (Mg++) from 0 and 0.5 mM, respectively, to 5.0 mM inhibited PTH-dependent (3 x 10(-9) M) cAMP production by 54 +/- 4% and 47 +/- 6%, respectively. The half maximal inhibitory concentration for both Ca++ and Mg++ was 0.9 mM. In addition, increasing extracellular barium (Ba++) or strontium (Sr++) from 0-10 mM inhibited PTH-dependent (3 x 10(-9) M) production by 54 +/- 7% and 62 +/- 6% with half of the maximal observed inhibition at 2.2 and 2.7 mM, respectively. The inhibition of PTH-dependent cAMP production by 2.5 mM Ca++ was not reversed by the calcium channel blockers diltiazem or verapamil (10(-4) M). However, changes in intracellular calcium may play some role in the inhibitory effects of Ca++ on cAMP production, since ionomycin (10(-6)-10(-5) M) lowered PTH-dependent cAMP production by 25-36%. Our data suggest that the proximal tubular cell can sense physiologically relevant changes in Ca++, providing a potential mechanism for the modulation of 1,25-dihydroxyvitamin D production or other tubular functions relevant to fluid and mineral homeostasis.     

Stimulation of cAMP accumulation and superoxide production in human neutrophils and monocytes

The effect of sodium fluoride (NaF) on superoxide generation and cyclic adenosine monophosphate (cAMP) levels in human neutrophils and monocytes was investigated. NaF (greater than 10 mM) stimulated superoxide (O2-) production in both cell types in a time dependent manner. NaF (0.5 to 20 mM) increased cAMP levels by 1.5- to 3.-fold in both neutrophils and monocytes. Increases in cAMP levels were time-dependent; the maximal level was attained within 5 minutes after the addition of NaF, and cAMP levels remained elevated for up to 10 minutes. Only high concentrations of NaF (10 and 20 mM) increased both cAMP levels and O2- production. Therefore, a direct role of cAMP in O2- generation is not likely. It is speculated that since NaF (greater than 10 mM) can complex with extracellular Ca++, and thus reduce free Ca++ concentration required for O2- generation, a NaF-dependent increase in cAMP may restore cytosolic free Ca++ by mobilizing intracellular stores of Ca++. Further, in view of the proposed involvement of a phosphorylation-dephosphorylation mechanism in the regulation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, we speculate that NaF, by inhibiting phosphoprotein phosphatase activity, may indirectly activate the NADPH oxidase system and thus superoxide generation.     

Mechanisms of action and differences in calcium channel blockers

Calcium ion (Ca++) serves an important role as an activation messenger; it initiates or regulates key cellular processes including contraction in the heart and vascular smooth muscle. Ca++ acts as both an electrical and a chemical signal. Upon entering the cell, the positively charged Ca++ carries an inward (depolarizing) current that contributes to pacemaker activity in the sinoatrial node and to atrioventricular conduction. Ca++ also binds to anionic surfaces of cell membranes and to anionic groups of both extracellular and intracellular proteins. The intracellular calcium-binding proteins include troponin and calmodulin, which when bound to Ca++ initiate contraction in cardiac and smooth muscles, respectively. Calcium channel blockers inhibit the entry of calcium into the cell, and thus prevent calcium from gaining access to the high-affinity, intracellular calcium-binding proteins. Verapamil and diltiazem decrease myocardial contractility and inhibit smooth muscle tone, while the dihydropyridines are mainly vasodilators. All of these drugs can play an important role in the treatment of hypertension.     

Influence of magnesium on calcium- and potassium-related responses in vascular smooth muscle.

The influence of Mg on Ca- and K-related responses of rabbit aortic strips was studied. Aortic strips bathed in either a K-free or 60.0 mM K solution were less sensitive to Ca in the presence of 1.2 mm Mg. Ca induced greater maximum tension in the aortic strips in the presence of Mg when bathed in 60.0 mM K. Addition of K to a K-free medium induced a slight relaxation at low concentrations (is less than 1.0 mm) which was Ca dependent. The relaxation was less in the presence of Mg. Wiolution were less sensitive to Ca in the presence of 1.2 mm Mg. Ca induced greater maximum tension in the aortic strips in the presence of Mg when bathed in 60.0 mm K. Addition of K to a K-free medium induced a slight relaxation at low concentrations (is less thth Mg in the medium throughout the experiment (90 min), the threshold concentration of K, including a contractile response, was increased as was the maximum tension. Acute addition of Mg decreased the maximum response to K. When K was lowered from 5.4 to 0.0 mm a contractile response, absent in the presence of Mg, occurred. When the K was increased, the tension returned to baseline. At 25-30 mm K, the tension again increased, reaching a maximum at 40-50 mm K. Ca uptake was stimulated at all K concentrations producing a contraction. Mg depressed this increase in uptake. 45Ca efflux was more rapid into a 50.0 mm than into 0.0 mm K solution. Mg delayed Ca efflux. It is concluded from these data that Mg has two components in its effect on the Ca associated with K responses. One is a competition with Ca at extracellular sites probably at the membrane, and the other is intracellular, probably a competition with Ca at sequestration sites.     

Calcium induced reversible alterations in excitation-contraction coupling in verapamil treated rat myocardium

We studied the effects of a calcium channel blocking agent, verapamil (V) (2 to 10 micrograms/ml), in the presence of increasing external calcium on simultaneously recorded transmembrane electrophysiological properties and mechanical function of rat myocardium. Left ventricular papillary muscles from male Fischer 344 rats were studied electrically, by standard microelectrode techniques, and mechanically in an isolated tissue bath at 30 degrees C. Control (0 micrograms/ml V + 2.4 mM Ca2+) = C, action potential duration at 50% and 75% repolarization (D50ap and D75ap) recorded from papillary muscles were short (14.1 +/- 0.75 ms; 33.3 +/- 2.7 ms) compared with recordings from papillary muscles subjected to increasing doses of verapamil (2, 4, 6, 8, or 10 micrograms/ml) + 2.4 mM Ca2+ = V, (17.3 +/- 0.77 ms; 121.4 +/- 8.9 ms: 10 micrograms/ml) (P less than 0.001). Upon augmentation of external calcium [10 micrograms/ml Verapamil + augmented Ca2+ (4.8, 7.2, or 9.6 mM] = VCa, D50ap and D75ap decreased but still remained significantly longer than control D50ap and D75ap (15.1 +/- 0.77 ms; 110.1 +/- 7.9 ms). Developed tension (Td), time to peak developed tension (TPT), time to one-half relaxation (T1/2R) and resting tension (Tr) decreased as a function of verapamil concentration. Although TPT and T1/2R returned toward C values when external calcium was increased, Tr continued to decrease while Td increased above control levels. A significant correlation was found between measured parameters of contraction and transmembrane action potential for C and VCa muscles. However, in V muscles no significant correlation was observed between these same mechanical and electrical parameters.(ABSTRACT TRUNCATED AT 250 WORDS)