EFFECTS OF NIFEDIPINE AND LOW [CA2-PIG PAPILLARY-MUSCLES(] ON MECHANICAL RESTITUTION DURING HYPOTHERMIA IN GUINEA)

Increasing the frequency of steady state stimulation increases the rat e of mechanical restitution in hypothermic (27-degrees-C) guinea pig p apillary muscles. In this paper we have investigated the influences of the calcium antagonist nifedipine and of reduced extracellular calciu m concentration on this phenomenon. We found that nifedipine abolished the frequency dependent increase in the restitution rate, which was a lso 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 I(Ca), the inward calci um current, which makes a larger than normal contribution to direct co ntractile activation in hypothermic myocardium.