The slow phase of the staircase in guinea-pig papillary muscle, influence of agents acting on transmembrane sodium flux.

Guinea-pig papillary muscles contracting at frequencies of 0.25 to 1 Hz after rest periods long enough to be followed by a rested-state contraction showed a biphasic staircase phenomenon. An initial phase was completed within about 6 beats (fast phase of the staircase). Then a slow phase followed reaching a steady state after about 4 min only. The effect on this slow phase exerted by drugs known to influence transmembrane Na-flux was investigated. Dihydroouabain in concentrations (1.5-5 X 10(-5) mol/l), causing no increase in the rested-state contraction considerably augmented the slow phase of the staircase thereby prolonging the time from the onset to the steady state by severalfold. The fast phase, however, remained unchanged as far as the steepening slow phase did not influence it. The rest decline of force of contraction, measured by repeated interruption of stimulation, was considerably prolonged by dihydroouabain (to about sixfold the control value by 5 X 10(-5) mol/l). However, dihydroouabain did not influence the time course by which the force of contraction decreased after lowering the [Ca]0 from 3.2 to 0.8 mmol/l. Stimulation of the muscles in Ca-free medium produced a transient increase in force of contraction as visualized by test contractions after addition of Ca. This positive inotropic aftereffect which depended on the frequency of stimulation in the Ca free solution was augmented severalfold by 1.5 to 3 X 10(-5) mol/l dihydroouabain.(ABSTRACT TRUNCATED AT 250 WORDS)