Ventricular relaxation in the stage 24 chick embryo following changes in volume and blockade of Na+ and Ca2+ channels

Abstract

Early in cardiac development, regulation of cytosolic Ca2+ has been thought to depend primarily on sarcolemmal Ca2+ transport. We hypothesized that perturbation of cytosolic Ca2+ in the embryonic ventricle would result in a change in ventricular relaxation which could be quantified by a monoexponential model. We reasoned that since it has been difficult to selectively block the Na(+)-Ca2+ exchanger in vivo, that blockade of Na+ (lidocaine) or Ca2+ (verapamil) channels in the embryonic heart may perturb cytosolic Ca2+ and thereby alter ventricular relaxation. All studies were performed in ovo in Hamilton-Hamburger stage 24 chick embryos. Isovolumic relaxation time (mean +/- standard deviation, 58 +/- 19 ms) was derived from dorsal aortic flow and atrioventricular inflow during 61 cardiac cycles in 4 embryos. Ventricular pressure was digitally recorded from 13 embryos during 188 cycles following intravenous injection of chick Ringer's solution (5 embryos), verapamil (4 embryos) or lidocaine (4 embryos). Ventricular relaxation was characterized by a monoexponential model: P(t) = P infinity + Poe-1/tau where P(t) = pressure at time (t), P infinity = pressure asymptote, Po = pressure at the onset of relaxation and tau = the isovolumic relaxation constant. Non-linear least-squares regression was used to estimate tau and P infinity during isovolumic relaxation at baseline and at 30 s and 60 s post-injection. Ventricular end-diastolic pressure was increased by all three interventions. Both lidocaine and verapamil prolonged cycle length. Lidocaine prolonged tau while chick Ringer's solution and verapamil did not. No significant change in P chi was observed. This study demonstrates that blockade of Na+ channels with lidocaine slows ventricular relaxation presumably by perturbing cytosolic Ca2+ via the Na(+)-Ca2+ exchange system. Changes following Ca2+ channel blockade with verapamil are less evident in the stage 24 chick embryo. Evaluation of ventricular relaxation may provide a useful way to study developmental aspects of Ca2+ transport.