Effect of cyanide and verapamil on sodium-free contractures in the frog heart

Sodium-free contractures were studied in myocardial strips from R. pipiens with extracellular sodium (Na0+) replaced by choline chloride and extracellular calcium (Ca20+) varied with EGTA buffer. At calculated Ca02+ below 2.8 X 10(-7) mol/l, no contracture occurred in most of the experiments, even in the presence of cyanide. When Ca02+ was above 2.8 X 10(-7) mol/l, relatively short tension transients of up to 80 sec duration could be avoided if the myocardial strip was previously equilibrated for 20 min in a Na+-Ca2+-free solution. Instead, contractures developed slowly within one to several hours. The maximum contracture was dependent on Ca02+ in a dose-response-like pattern. The time-course of contracture development was not affected by verapamil, but KCN significantly increased the rate of resting tension increase. In solutions with normal Na+-Ca2+ content and even in a Na+-Ca2+-free milieu, the cellular ultrastructure was normal. Development of contracture after addition of Ca2+ to the Na+-free solution was combined with ultrastructural damage of the ventricular strip. It is concluded that Na+-free contractures depend on transsarcolemmal net-Ca2+ uptake as a sum of Na-Ca-exchange-dependent Ca2+ uptake and active sequestering of intracellular free calcium Ca2+ mediated by sarcolemmal and probably intracellular Ca2+-ATPases. The negative inotropic effect of the Ca blocker verapamil seems not to be mediated by the Na-Ca exchange.     

Effect of acetylcholine on ion transport in the frog skeletal muscle

1. The effect of acetylcholine (ACh) on the ion transport of frog (Rana esculenta) sartorius muscles was studied. ACh was applied in bathing solution, Na influx and K efflux were measured using 24Na and 42K isotopes. 2. Na influx of sartorius muscles was increased by 1 mmol/1 ACh 2-10 fold depending on the experimental arrangement. The increase was greater if Na influx was measured at the beginning of ACh depolarization. During ACh treatment the Na influx took about the same time course as the depolarization recorded extracellularly. This type of recording approximately reflects the depolarization proceeding on the sartorius muscle fibres. 3. The presence of 31 nmol/l tetrodotoxin (TTX) did not modify the degree of increase of Na influx. 4. Rate coefficients for K efflux were increased 2-5 fold by ACh. The maximum rate coefficients were obtained in the first minute of ACh treatment. 5. Increase in K loss evolves also in the presence of 31 nmol/l TTX. The increase in rate coefficients was found to be about 30% less than without TTX in the first minute of ACh action. 6. The results indicate that in the presence of ACh the observed increase in Na influx and K efflux is brought about mainly by changes in Na and K conductance induced by ACh at the end-plates rather than by the action potentials accompanying ACh depolarization.     

Effect of primycin on some electric properties of the frog skeletal muscle

The effect of primycin, a guanidine-type antibiotic was studied on the electric properties and 42K+ uptake of the frog sartorius and semitendinosus muscle. Both in normal and choline chloride Ringer solution, primycin evoked a concentration and time dependent depolarization of the surface membrane of the muscle. This depolarization was significantly increased by Na ions. Primycin treatment was shown to evoke a dose-dependent decrease of the depolarization induced by 20 mM K+-Ringer. When the muscles were incubated in a Ringer solution containing choline chloride, during an incubation period of 30 min the uptake of 42K+ was decreased to 12% upon the exposure to 5 x 10(-6) mol primycin as compared to the control value. As the primycin-induced depolarization increased, the shape and amplitude of the action potentials elicited by square-wave electric impulses were altered and decreased, respectively. In sodium isaethionate Ringer 1--2 x 10(-6) M primycin induced a slow depolarization resulting in firing potentials. The results suggest that primycin depolarizes the surface membrane exclusively through the blockade of the resting K+ channels, the other phenomena being the results of this depolarizing effect.     

Effect of ouabain on electrical activity of the isolated frog muscle spindle

Changes in electrical activity of the isolated frog muscle spindle were studied in Ringer's solution containing ouabain. The presence of ouabain in the solution increased the spontaneous firing rate of the receptors up to a maximum and then reduced it quickly to zero. The amplitude of the action potentials was reduced on the average to 40% of normal. Ouabain causes initial disappearance of the hyperpolarization phase of the receptor potential and a subsequent decrease in amplitude of its dynamic phase to zero. The decrease in amplitude of the receptor potential and action potential and also the changes in firing rate in the solution with ouabain depend on the frequency of their spontaneous activity. The changes observed can be explained by depolarization of the membrane of the nerve endings and the first node of Ranvier, developing as a result of blocking of the sodium pump by ouabain.Translated from Neirofiziologiya, Vol. 5, No. 6, pp. 576–582, November–December, 1973.     

Effect of hypertonic solutions on electrical activity of the isolated frog muscle spindle

Changes in electrical activity of the isolated frog muscle spindle were investigated in hypertonic solutions obtained by adding 400 mM sucrose, glucose, or glycerol to Ringer's solution. The spontaneous firing rate in hypertonic sucrose and glucose solutions increased at first (for 3–5 min) and then fell rapidly to zero; the receptor potential and evoked spike activity diminished under these conditions and disappeared. In the hypertonic solution with glycerol a similar effect was observed but, unlike in the first two media, in this case spike activity returned after its initial increase to the normal level; a second rise in the firing rate was then observed up to a steady value which was higher than normal. After rinsing out the hypertonic sucrose and glucose solutions with ordinary Ringer's solution the spontaneous and evoked activity gradually returned to normal with a small overshoot. During the rinsing out of the hypertonic glycerol solution a sharp and considerable rise in spontaneous activity was first observed, while the changes in frequency of the evoked activity were negligible. The spike activity then returned to normal. The observed changes in electrical activity of the muscle spindle in hypertonic media are attributed to deformation of the sensory terminals and intrafusal muscle fibers (in the glycerol medium), leading to depolarization of the receptor membrane.P. K. Anokhin Institute of Normal Physiology, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 8, No. 3, pp. 291–299, May–June, 1976.