Ryanodine does not affect calcium current in guinea pig ventricular myocytes in which Ca2+ is buffered.
Calcium current in mammalian ventricular muscle is altered in the presence of ryanodine. Previous studies performed on rat ventricular cells have shown a slowing of Ca2+ current inactivation and suggest the hypothesis that ryanodine, by reducing the release of Ca2+ from the sarcoplasmic reticulum, reduces the availability of Ca2+ for inactivation of Ca2+ current (Ca(2+)-dependent inactivation). Another hypothesis is that the effects of ryanodine on Ca2+ current are due to a mechanical connection of the ryanodine receptor with the L-type Ca2+ channel. To further test these hypotheses we examined the effect of ryanodine on Ca2+ current in single voltage-clamped guinea pig ventricular myocytes that contained Ca2+ indicator and Ca2+ buffer. We used fura 2 (pentapotassium salt) to confirm that the ryanodine we used was capable of abolishing Ca2+ release from the sarcoplasmic reticulum during the period in which it was present. We perfused the cells with 10 mM EGTA to block changes in intracellular Ca2+ concentration. In the absence of internal EGTA, Ca2+ currents displayed biexponential inactivation and Ca(2+)-dependent inactivation (steady-state inactivation curves turned up at positive potentials). Inactivation was slowed by ryanodine at 10 microM. In cells perfused internally with EGTA, however, ryanodine had no effects, and steady-state inactivation curves were not shifted to the right. We conclude that, in guinea pig ventricular myocytes, the effects of ryanodine on Ca2+ current are mediated by Ca2+ and thus the effects of ryanodine do not provide a basis on which to postulate a physical connection between the L-type Ca2+ channel and the ryanodine receptor (sarcoplasmic reticulum Ca2+ release channel).
Balke
C W
CW
Department of Physiology, University of Maryland, School of Medicine, Baltimore 21201.
Wier
W G
WG
D000818:Animals; D002021:Buffers; D002118:Calcium / Q000378:metabolism / Q000502:physiology*; D004533:Egtazic Acid / Q000494:pharmacology; D004553:Electric Conductivity; D006168:Guinea Pigs; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D009994:Osmolar Concentration; D012433:Ryanodine / Q000494:pharmacology*; D012964:Sodium / Q000494:pharmacology; D012996:Solutions
D002021:Buffers; D012996:Solutions; D012433:Ryanodine; D004533:Egtazic Acid; D012964:Sodium; D002118:Calcium