Effects of Osmolarity Change on the Excitation-Contraction Coupling of Bullfrog Ventricle

Abstract

Summary The effect of hyper-and hypotonic solutions on the electrical as well as the mechanical activities of the bullfrog ventricle were investigated. Hypertonic solutions up to 400 % tonicity, made by adding sucrose, showed two kinds of inhibitory effect; i.e. a consistent, rapid shortening of the action potential duration and a slowly progressing suppression of contractile tension. The inhibitory effect of the hypertonic solution up to 300 %, made by adding NaCl, was more pronounced on the contractile process than on the electrical phenomena. In this condition, the inhibition of the twitch tension occurred with two time constants. The first, rapid decline seems to be related to the suppressive effect on some superficial site while the slower decreasing phase showed a similar time course as the development of contracture. Hypotonic (33.3 % and 12.5 %) perfusion revealed triphasic changes in the time course of single twitch without accompanying any parallel change in the action potential. From these results it was concluded that the bullfrog ventricular muscle has some superficial site for Ca regulation which is very sensitive to osmolarity change.It is known that the twitch contraction of skeletal muscle can be strongly suppressed in hypertonic media without any essential change in the action potential (HODGKIN and HOROWICZ, 1957;HOWARTH, 1958). It has been shown that the caffeine-induced contracture is also inhibited by hypertonic perfusion (ISA-ACSON, 1969). The mechanism of the contractile inhibition by hypertonicity is still unknown although a direct suppressive effect on the contractile elements is postulated by many authors.Recently LANNERGREN and NOTH (1973) have reported that the hypertonic

Mechanism of inotropic action by hypotonic solution in the frog atrial muscle.

“…The increase in muscle weight occurred with the time course corresponding to the slow decrease in tension, and muscle weight increased 10% during the 30 min perfusion of hypotonic solutions. In the bullfrog ventricle, a 20 % increase was obtained when exposed to the hypotonic solution (down to 50% of normal tonicity) (KAWATA et al, 1974). The muscle preparation used for the measurement of weight was composed of multifibers, thus it might take more time for diffusion than in the muscle fibers used for tension measurements.…”

“…A reduced [K+]i may account for the depolarization of the cell membrane at a late stage of perfusion. Depolarization by about 25 mY with the hypotonic solution (12.5 % of normal tonicity) in the frog ventricle (KAWATA et al, 1974) and by 5 mY (67 % of normal tonicity) in the rabbit papillary muscle (AKIYAMA and FOZZARD,1975) were shown. In the present experiment, since the depolarization was small it is not plausible to cause inactivation of the Na channel and to reduce the excitability of muscle cells.…”

“…The shortening of action potential duration may contribute to the decline in twitch by the exposure to the hypotonic solution. Although the mechanism of this shortening was not clarified, the same amount of shortening is observed on the frog ventricle (KAWATA et al, 1974). In frog ventricle muscle fibers, neither hypetonic nor hypotonic solutions reduce the level of plateau component of the action potential (HERMSMEYER et al, 1972;KAWATA et al, 1974).…”

“…of heart muscle, although the effects are complicated, an initial phasic increase of twitch which then decreases below the original level was observed (KAWATA et al, 1974;CHAPMAN, 1978). The application of strongly hypotonic solutions evoked a contracture (KAWATA and KAWAGQE, 1975) and in Na-free fluid the contracture induced by reducing the tonicity increased in size (CHAPMAN, 1978).…”

Excitation of Squid Giant Axons in Hypotonic and Hypertonic Solutions

“…Use of isotonic solutions alone limits the range of ionic concentration changes that can be made. Until now, many investigators have examined the excitability of nerves (MULLER-MOHNSSEN and BARSKE, 1974) or muscle (HODGKIN and HOROWICZ, 1957;KAWATA et al, 1974;SUAREZ-KURTZ and SORENSON, 1977) in hypotonic and hypertonic solutions. Since they used intact cells, it was difficult to separate the effect of the intended ionic concentration changes from those of changes in cell volume, in cell surface area and in ionic concentrations inside the cell (FREEMAN et al, 1966;SPYROPOULOS, 1977).…”