Effects of nifedipine and low [Ca2+] on mechanical restitution during hypothermia in guinea pig papillary muscles.

Increasing the frequency of steady state stimulation increases the rate of mechanical restitution in hypothermic (27 degrees C) guinea pig papillary muscles. In this paper we have investigated the influences of the calcium antagonist nifedipine and of reduced extracellular calcium concentration on this phenomenon. We found that nifedipine abolished the frequency dependent increase in the restitution rate, which was also sensitive to extracellular [Ca2+]. These findings suggest that the level of intracellular [Ca2+] can influence the rate of restitution. It is implied that this effect is mediated via ICa, the inward calcium current, which makes a larger than normal contribution to direct contractile activation in hypothermic myocardium.