In depolarized guinea pig myocardium, automatic activity development is associated with a time-dependent decrease in outward K+ current and an increase in background inward Ca2+/Na+ current.
Membrane potential was changed uniformly in segments, 0.7-1.0 mm long, of guinea pig papillary muscles excised from the right ventricle by using extracellular polarizing current pulses applied across two electrically insulated cf preparations superfused with Tyrode's solution at maximum diastolic membrane potentials ranging from-35.2+/-7.5 (threshold) to +4.0+/-9.2 mV. The average maximum dV/dt of RAD ranged from 17.1 to 18.0 V/sec within a membrane potential range of -40 to +20 mV. Raising extracellular Ca2+ concentration Ca2+0 from 1.8 to 6.8 mM, or application of isoproterenol (10(-6)g/ml) enhanced the rate of RAD, but lowering Ca2+0 to 0.4 mM or exposure to MnCl2 (6 mM) abolished RAD. RAD were enhanced by lowering extracellular K+ concentration K+0 from 5.4 to 1.5 mM. RAD were suppressed in 40% of fibers by raising K+0 to 15.4 mM, and in all fibers by raising K+0 to 40.4 mM. This suppression was due to increased K+0 and not to K-induced depolarization because it persisted when membrane potential was held by means of a conditioning hyperpolarizing puled gradually after maximum repolarization. These observations suggest that the development of RAD in depolarized myocardium is associated with a time-dependent decrease in outward current (probably K current) and with increase in the background inward current, presumably flowing through the slow cha-nel carrying Ca or Na ions, or both.
Imanishi et al. (Wed,) conducted a other in Depolarized guinea pig ventricular myocardium. Extracellular polarizing current pulses and ion concentration changes was evaluated on Characteristics and mechanisms of automatic activity (RAD). In depolarized guinea pig myocardium, automatic activity development is associated with a time-dependent decrease in outward K+ current and an increase in background inward Ca2+/Na+ current.