Effect of internal sodium and cellular calcium load on voltage-dependence of the Indo-1 transient in guinea-pig ventricular myocytes.
OBJECTIVE
To investigate the effect of altering internal Na and cell Ca load on the voltage-dependence of the intracellular Ca transient.


METHODS
Ventricular myocytes were isolated enzymatically from the guinea-pig heart. They were patch clamped and dialysed internally with pipette solutions which contained either 0 or 10 or 20 mM Na. Intracellular Ca was monitored with Indo-1 and experiments were carried out at 36 degrees C. A standard level of Ca loading was established before each test pulse by applying a train of conditioning pulses. The voltage-dependence of the Cai (Indo-1) transient provided information about the mechanisms which trigger Ca release from the sarcoplasmic reticulum (SR).


RESULTS
The voltage-dependence of L-type Ca current (ICa.L) was assessed in separate experiments by dialysing myocytes with a Cs-based solution. ICa.L had a maximum amplitude at 0 mV, declined at more positive potentials and there was little net inward ICa.L at +100 mV. The rapid initial phasic component of the Indo-1 transient was abolished by ryanodine/thapsigargin; therefore, this component reflected the magnitude of SR Ca release. In cells dialysed with 10 mM Na, the voltage-dependence of the Indo-1 transient was different from ICa.L. The Indo-1 transient became maximal at +20 mV, and the decline of the Indo-1 transient at positive potentials was less steep than the decline of ICa.L. A large proportion of the phasic Indo-1 transient could remain at positive potentials where there was no detectable ICa.L. Increasing dialysing Na from 10 to 20 mM led to a marked change in voltage-dependence of the Indo-1 transient. With 20 mM Na, the amplitude of the phasic Indo-1 transient remained large between +20 and +100 mV. Removing Na from the pipette dialysis solution had the opposite effect on voltage-dependence of the transient. For each dialysing [Na], the level of cellular and SR Ca content was altered by varying the potential of conditioning pulses applied before each test pulse. There was no significant effect on voltage-dependence of the Indo-1 transient of either increasing or reducing the cellular Ca content.


CONCLUSIONS
These data are consistent with the hypothesis that the voltage-dependence of the Cai transient results from the sum of the voltage-dependencies of the two main trigger mechanisms--Ica.L and reverse Na/Ca exchange. When a myocyte was dialysed with Na-free solution, the voltage-dependence of the Cai transient became more similar (but not identical) to that for ICa.L. With 20 mM Na dialysis, the altered voltage-dependence of the Cai transient may reflect an increased trigger influence of reverse Na/Ca exchange.
Levi
A J
AJ
Department of Physiology, University of Bristol, UK.
Li
J
J
Litwin
S E
SE
Spitzer
K W
KW
D000818:Animals; D000889:Anti-Arrhythmia Agents / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D006168:Guinea Pigs; D066298:In Vitro Techniques; D007211:Indoles / Q000378:metabolism; D008564:Membrane Potentials; D009206:Myocardium / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D012433:Ryanodine / Q000494:pharmacology; D012519:Sarcoplasmic Reticulum / Q000378:metabolism; D012964:Sodium / Q000378:metabolism*; D019284:Thapsigargin / Q000494:pharmacology
D000889:Anti-Arrhythmia Agents; D007211:Indoles; D012433:Ryanodine; D019284:Thapsigargin; D012964:Sodium; D000252:Calcium-Transporting ATPases; C048960:indo-1; D002118:Calcium