Comparison of ultra-slow, voltage-dependent inactivation of the cardiac L-type Ca2+ channel with Ca2+ or Ba2+ as the charge carrier in ferret ventricular myocytes.
The whole-cell patch clamp technique was used to investigate the effect of different charge carriers upon ultra-slow voltage-dependent inactivation of L-type Ca2+ channel current in ferret ventricular myocytes at 37 degrees C. Intracellular Ca2+ was buffered with 10 mM EGTA and the membrane potential held at -40 mV. With Ba2+ as the charge carrier, the L-type current decayed throughout 20 s pulses to 0 mV as a result of ultra-slow voltage-dependent inactivation. In contrast, with Ca2+ as the charge carrier, there was no such slow decay of the current as the current decayed almost completely in the first approximately 100 ms as a result of Ca(2+)-dependent inactivation. However, with Ca2+ as the charge carrier it is still possible that ultra-slow voltage-dependent inactivation occurs. A conditioning-test pulse protocol and a second protocol were used to test for the development of ultra-slow inactivation during 20 or 30 s pulses to 0 mV with Ca2+ as the charge carrier. Ultra-slow inactivation did occur and it was qualitatively similar to that with Ba2+ as the charge carrier. The onset of ultra-slow inactivation with Ca2+ as the charge carrier could be described by the sum of two exponentials with time constants of 0.3 and 6.7 s. Recovery from ultra-slow inactivation with Ca2+ as the charge carrier was also measured with a conditioning-test pulse protocol and was best described by the sum of two exponentials with time constants of 0.5 and 6.2 s. We conclude that ultra-slow inactivation of the L-type current does occur with the physiological charge carrier, Ca2+, but it is normally masked by Ca(2+)-dependent inactivation.
McMorn
S O
SO
Department of Physiology, University of Leeds, UK.
Harrison
S M
SM
Zang
W J
WJ
Boyett
M R
MR
D000818:Animals; D001464:Barium / Q000494:pharmacology*; D002118:Calcium / Q000494:pharmacology*; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism*; D005260:Female; D005289:Ferrets / Q000378:metabolism*; D006352:Heart Ventricles / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D066298:In Vitro Techniques; D008297:Male; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D018408:Patch-Clamp Techniques
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D001464:Barium; D002118:Calcium