ryanodine and cobaltous-chloride

The effects of caffeine (10 mM) on depolarization-activated, calcium-independent outward K+ currents were investigated in isolated rat ventricular myocytes, using whole-cell clamping. The external solution contained CoCl2 2 mM and the internal solution contained ethylene glycol-bis(-aminoethyl ether) N,N,N',N'-tetraacetic acid 10 mM. Caffeine decreased the peak amplitude of the total current and the sustained plateau current. Caffeine did not modify the steady state inactivation curve, which was fitted by two Boltzmann functions. Caffeine blocked the tetraethylammonium-sensitive slowly activating and inactivating outward current by 32% and the 4-aminopyridine-sensitive rapidly activating and inactivating transient outward current by 19%. Caffeine did not modify the inactivation rate or the time course of the recovery from inactivation of the transient current. Ryanodine 10 microM did not modify any of the current components and the effect of caffeine was not modified by ryanodine pretreatment. The phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine 100 microM, did not modify the depolarization-activated calcium-independent outward currents.

Topics: Action Potentials; Animals; Caffeine; Calcium; Cobalt; Egtazic Acid; Electrophysiology; Heart; Heart Ventricles; In Vitro Techniques; Myocardium; Potassium; Rats; Ryanodine

The aim of the study was to investigate the effects of chronic diabetes mellitus on electromechanical properties of ventricular papillary muscles.. Conventional glass microelectrodes and tension recording techniques were used in isolated hearts of rats made diabetic for 30-40 weeks by single intravenous injections of streptozotocin.. Experimental animals were male Wistar rats of 200-250 g. Diabetic rats (n = 14) were given streptozotocin 65 mg.kg-1; controls (n = 15) were given vehicle only.. (1) The maximum upstroke velocity of the action potential duration of diabetic muscles was decreased compared to control, with no difference in the resting potential. (2) At all stimulation frequencies (0.2, 1 and 5 Hz), and particularly the lower ones, the action potential duration of diabetic muscles was longer than control. (3) In diabetic muscles, frequency dependent shortening of the late phase of action potential duration (APD75, APD90) was more pronounced, and frequency dependent lengthening of the early phase (APD25, APD50) was less pronounced. (4) A blocker of transient outward current, 4-aminopyridine, lengthened the early phase of action potential durations by the same amount in diabetic and control muscles. (5) A Ca2+ channel blocker, CoCl2, dramatically shortened all levels of action potential duration, with much greater effect on diabetic muscles. (6) Ryanodine lengthened the early phase of action potential duration and shortened the late phase in both diabetic and control muscles. It enhanced the difference between the groups in the early phase. (7) Developed tension in the presence of ryanodine (ryanodine resistant tension component) was greater in diabetic muscles than in control.. The findings suggest that altered Ca2+ current, but not altered Na(+)-Ca2+ exchange current or altered transient outward current, significantly prolongs action potential duration in diabetic rat ventricular muscles.

Topics: 4-Aminopyridine; Action Potentials; Animals; Calcium Channel Blockers; Calcium Channels; Cobalt; Diabetes Mellitus, Experimental; Electric Stimulation; Heart; In Vitro Techniques; Male; Myocardial Contraction; Myocardium; Papillary Muscles; Rats; Rats, Inbred Strains; Ryanodine

Transient changes of extracellular free calcium in rabbit ventricular muscle under nonsteady state conditions were measured with double-barreled calcium microelectrodes. Resumption of stimulation after a rest interval produces a cumulative decrease of extracellular free calcium often by more than 10% (with bulk extracellular free calcium = 0.2 mM). The extracellular free calcium returns to the bulk value as a new steady state is achieved. The changes of extracellular free calcium recorded presumably represent net calcium uptake and loss by cardiac muscle cells. These cumulative extracellular free calcium depletions are blocked by 0.5 mM cobalt and 1 microM nifedipine and are increased to 167 +/- 11% of control by the calcium agonist Bay k 8644 (1 microM) and to 620 +/- 150% of control by increasing stimulus frequency from 0.2-2 Hz. Caffeine (10 mM) inhibits the cumulative extracellular free calcium depletions, probably by rendering the sarcoplasmic reticulum unable to accumulate calcium. It is proposed that the extracellular free calcium depletions recorded represent, in large part, calcium which has entered the cells and has been taken up by the sarcoplasmic reticulum (which had become depleted of calcium during the rest interval). Nifedipine and cobalt inhibit these cumulative depletions presumably by preventing the calcium entry which could subsequently be accumulated by the sarcoplasmic reticulum. The net cellular calcium uptake produced by such a post-rest stimulation protocol can also be inhibited by 1-3 microM acetylstrophanthidin and reduction of extracellular sodium to 70 mM. Acetylstrophanthidin and low extracellular sodium would be expected to shift the sodium-calcium exchange in favor of increased calcium uptake, which may, in turn, prevent the loss of sarcoplasmic reticulum calcium during the rest interval. This would limit the amount of calcium which the sarcoplasmic reticulum could take up with subsequent activation. In contrast to the results with caffeine, ryanodine (1 microM) increases the magnitude and rate of calcium uptake after a rest interval, indicative of a fundamental difference in the actions of caffeine and ryanodine. When stimulation is stopped in the presence of ryanodine, extracellular free calcium increases much faster than in control. This suggests that ryanodine may enhance calcium uptake by the sarcoplasmic reticulum during repetitive stimulation and may enhance calcium efflux from the sarcoplasmic reticulum during quies

Topics: 3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester; Animals; Caffeine; Calcium; Cobalt; Electric Stimulation; Extracellular Space; Heart; In Vitro Techniques; Microelectrodes; Myocardium; Nifedipine; Rabbits; Ryanodine; Strophanthidin

To determine whether Na+-Ca2+ exchange is a physiologically significant Ca2+ efflux mechanism in rabbit ventricle, we investigated the effects exerted on postrest contractions by interventions that alter the transmembrane distribution of Na+ or Ca2+ so as to retard Ca2+ efflux via this system. Contractions elicited after rest periods of 0.25-10 min in duration were studied. The following interventions increased postrest contractions much more than those elicited by rhythmic stimulation: 1) Na+ pump inhibition by cardiac glycosides or by a reduction in extracellular K+, 2) reduction of extracellular Na+ (maintaining a constant [Ca2+]-to-[Na+]2 ratio), and 3) elevation of extracellular Ca2+. In contrast, isoproterenol, norepinephrine, and histamine produced comparable increases in both rhythmically stimulated and postrest contractions, suggesting that the postrest contractile potentiation was not just the result of a general increase in inotropic state. Ryanodine, which appears to antagonize sarcoplasmic reticulum (SR) Ca2+ release in cardiac muscle, markedly reduced the amplitude of the postrest contractions, but only modestly decreased rhythmically stimulated responses. Results suggest 1) that Ca2+ released from SR is involved in postrest response, 2) that Na+-Ca2+ exchange serves as a Ca2+ efflux pathway in normally polarized resting rabbit ventricle, and 3) that this activity in part determines the amount of Ca2+ available for release from SR.

Topics: Animals; Calcium; Cobalt; Heart; In Vitro Techniques; Male; Myocardial Contraction; Myocardium; Periodicity; Rabbits; Ryanodine; Sarcoplasmic Reticulum; Sodium; Strophanthidin