Cycle length-dependent action potential duration and contractile force at various [Ca2+]o in guinea pig papillary...

This study was designed to determine the role of calcium in the cycle length-dependent changes in the action potential duration (APD) and the contractile force in guinea pig papillary muscle. APD correlated with the contractile force during the steady state, with [Ca2+]o ranging from 0.3-7.2 mM. High [Ca2+]o increased the force and shortened APD, while low [Ca2+]o had the opposite effect. During the steady state, as the cycle length of stimulation was decreased, the increase in the contractile force was inversely related to the increase in APD within a [Ca2+]o range of 0.9-5.4 mM and at diastolic intervals of < or = 600 msec. At longer diastolic intervals, the relationship between changes in contractile force and changes in APD was variable and non-linear at each [Ca2+]o. Changes in the postextrasystolic force were not related to APD. The phenomenon of APD overshoot i.e., an APD that was longer during short than during basic cycles, was observed at < 5.4 mM [Ca2+]o and was most pronounced at the lowest [Ca2+]o. Assuming that the contractile force reflects [Ca2+]i, we concluded that the cycle length-dependent APD curve during the steady state is influenced by [Ca2+]i in a manner consistent with the inverse relationship between APD and the contractile force.