[Contraction-relaxation dynamics and mechanical restitution in the developing myocardium of the chick embryo].

Parameters of the contraction-relaxation cycle and mechanical restitution (MR) were assessed in isolated ventricular preparations of 3- and 4-day chick embryos (EM) and posthatched (PH) chicks. Ryanodine reduced the relaxation rate in the EM but increased it in the PH chicks. It also suppressed a rest-induced potentiation and the MR in all the preparations. Low Na superfusion significantly suppressed the relaxation and decreased the rest potentiation in the myocardial preparations all ages. The findings substantiate existence of a sarcolemmal Ca pool which participates in regulation of twitch parameters and Ca outflux via the Na-Ca exchange.