Ryanodine does not affect calcium current in guinea pig ventricular
myocytes in which Ca$^2+$ is buffered.
Circ. Res., 68 (3):
897--902 (March 1991)Abstract
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 Ca$^2+$ current inactivation and
suggest the hypothesis that ryanodine, by reducing the release of
Ca$^2+$ from the sarcoplasmic reticulum, reduces the availability
of Ca$^2+$ for inactivation of Ca$^2+$ current (Ca$^2+$-dependent
inactivation). Another hypothesis is that the effects of ryanodine
on Ca$^2+$ current are due to a mechanical connection of the
ryanodine receptor with the L-type Ca$^2+$ channel. To further
test these hypotheses we examined the effect of ryanodine on Ca$^2+$
current in single voltage-clamped guinea pig ventricular myocytes
that contained Ca$^2+$ indicator and Ca$^2+$ buffer. We used
fura 2 (pentapotassium salt) to confirm that the ryanodine we used
was capable of abolishing Ca$^2+$ 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 Ca$^2+$
concentration. In the absence of internal EGTA, Ca$^2+$ 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 Ca$^2+$
current are mediated by Ca$^2+$ and thus the effects of ryanodine
do not provide a basis on which to postulate a physical connection
between the L-type Ca$^2+$ channel and the ryanodine receptor
(sarcoplasmic reticulum Ca$^2+$ release channel).
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