Rate-tension staircase in rabbit ventricular muscle: relation to ionic exchange

Abstract

In rabbit papillary muscle, for a given rate of contraction, there is a critical temperature at which systolic tension is maximally developed; at or below this temperature an increase in rate results in either no further increase or a fall in tension. Four kinetically defined phases of 45ca exchange (0-3) were found in the arterially perfused rabbit ventricular septum. A fall in temperature accompanied by positive inotropism Is associated with an increased Ca++ content In phase 2. In zero Ca++ perfusate, the rate constant of decline of developed tension of a papillary muscle beating at a stable rate is virtually the same as the rate constant of phase 2. A theory for the mechanism of the classical Bowditch staircase is proposed whereby an increased rate of contraction Is accompanied by a "Na-pump lag." This allows Ca++ to increase at sites (represented by phase 2) of Na+-Ca+ competition with resultant inotropism on successive beats. K+ loss is not fundamental to the development of the rate-tension staircase, but is secondary to Na+ movements which in turn control subsequent Ca++ movements and, thereby, tension development.