Ca2+ uptake by the sarcoplasmic reticulum decreases the amplitude of depolarization-dependent [Ca2+]i transients in rat gastric myocytes.
Gastric myocytes loaded with fura-2 were voltage-clamped at -60 mV. Depolarizations to 0 mV evoked nifedipine-sensitive (5 microM) inward currents and Ca2+ transients. Cyclopiazonic acid (5 microM) elevated steady-state [Ca2+]i and reduced Ca current (ICa), but when divalent cations were omitted from the extracellular solution, cyclopiazonic acid had no effect on either the amplitude or the current-voltage relationship of the nifedipine-sensitive current. This suggests that the reduction in ICa was caused by the rise in steady-state [Ca2+]i. The relationship between the total Ca2+ influx carried by the Ca2+ current (sigmaI(Ca).dt) and the amplitude of the Ca2+ transient (delta[Ca2+]i) was analysed for experiments using physiological Ca2+ solutions by calculating the ratio delta[Ca2+]i/sigmaI(Ca).dt. Cyclopiazonic acid (5 microM) and ryanodine (10 microM) both increased this ratio, indicating a decrease in the buffering power of the cell. Mimicking the increase in steady-state [Ca2+]i produced by these agents by changing the holding potential to -40 mV, however, did not affect delta[Ca2+]i/sigmaI(Ca).dt. It was concluded that up-take by a ryanodine-sensitive store normally limits Ca2+ distribution to the bulk cytoplasm following entry to the cell through dihydropyridine-sensitive channels.
White
C
C
Department of Physiology, Medical Biology Centre, Queen's University of Belfast, UK.
McGeown
J G
JG
D000818:Animals; D002118:Calcium / Q000493:pharmacokinetics*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D007211:Indoles / Q000494:pharmacology; D008274:Magnesium / Q000494:pharmacology; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D018485:Muscle Fibers, Skeletal / Q000737:chemistry / Q000201:enzymology; D009130:Muscle, Smooth / Q000166:cytology / Q000502:physiology*; D009543:Nifedipine / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D051381:Rats; D017207:Rats, Sprague-Dawley; D012433:Ryanodine / Q000494:pharmacology; D019837:Ryanodine Receptor Calcium Release Channel / Q000378:metabolism; D012519:Sarcoplasmic Reticulum / Q000737:chemistry / Q000201:enzymology*; D013270:Stomach / Q000166:cytology / Q000502:physiology*
D002121:Calcium Channel Blockers; D004791:Enzyme Inhibitors; D007211:Indoles; D019837:Ryanodine Receptor Calcium Release Channel; D012433:Ryanodine; D000252:Calcium-Transporting ATPases; D008274:Magnesium; D009543:Nifedipine; D002118:Calcium; C000543:cyclopiazonic acid