RELATION BETWEEN SPONTANEOUS CONTRACTION AND SARCOPLASMIC-RETICULUM FUNCTION IN CULTURED NEONATAL RAT CARDIAC MYOCYTES

The relation between spontaneous contraction, Ca2+ oscillations, and s arcoplasmic reticulum (SR) function was studied in cultured neonatal r at cardiac myocytes. Spontaneous contraction and Ca2+ oscillations wer e irregular at day 2 of culture but became regular at day 6 of culture in neonatal rat cardiac myocytes. The rate of spontaneous contraction and the frequency of Ca2+ oscillations were decreased by verapamil an d were abolished in the absence of extracellular Ca2+ at both day 2 an d day 6 of culture. Ryanodine and thapsigargin increased the rate of c ontraction and the frequency of Ca2+ oscillations at day 2 of culture but did not affect contractions and Ca2+ oscillations at day 6 of cult ure. Ultrastructural observation showed that the structure of SR devel oped less at day 6 of culture. The present results suggest that sponta neous contraction and Ca2+ oscillations are due mainly to extracellula r Ca2+ influx but not to Ca2+ release from SR in neonatal rat cardiac myocytes. (C) 1998 Elsevier Science Inc.