AJP - Cell Physiology, Vol 271, Issue 3 C833-C841, Copyright © 1996 by American Physiological Society

ARTICLES

Buffering of plasmalemmal Ca2+ current by sarcoplasmic reticulum of guinea pig urinary bladder myocytes

A. Yoshikawa, C. van Breemen and G. Isenberg
Department of Cardiology, Wakayama Medical College, Japan.

The effects of cyclopiazonic acid (CPA), an inhibitor of the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA), on cytosolic Ca2+ concentration ([Ca2+]c) and membrane currents were studied in isolated urinary bladder myocytes to test the hypothesis that the sarcoplasmic reticulum (SR) buffers Ca2+, which enters the myocyte at a slow to moderate rate. Inhibition of SERCA by CPA was demonstrated by the following modifications of the caffeine-induced [Ca2+]c transients: 1) CPA prolonged the 90% decay time from peak to resting [Ca2+]c from 2.2 +/- 0.3 to 8.3 +/- 0.92 s (n = 5), 2) CPA abolished the "undershoot" of the [Ca2+]c transient that follows the washout of caffeine, and 3) CPA prevented caffeine from inducing a second [Ca2+]c transient. CPA reversibly increased resting [Ca2+]c. Starting from a control [Ca2+]c of 137 +/- 10 nM, 19 of 24 cells responded with a monotonic increase in [Ca2+]c to a steady [Ca2+]c of 238 +/- 10 nM, whereas 5 of 24 cells responded with a transient rise of [Ca2+]c to 472 nM (within 2.8 +/- 0.5 s) followed by a decay to a steady [Ca2+]c of 161 +/- 10 nM. The CPA-mediated rise in [Ca2+]c was augmented by increasing extracellular Ca2+ concentration ([Ca2+]o), suggesting a "leakage pathway" for Ca2+ influx that is unmasked by SERCA blockade. CPA reduced [Ca2+]c transients and Ca(2+)-activated K+ currents (IK,Ca), induced by depolarizing clamp steps from -60 to 0 mV, compatible with suppression of SR Ca2+ release on depletion of SR Ca2+. To reduce the contribution due to Ca(2+)-induced Ca2+ release, the cells were depolarized with a slow ramplike command (-60 to 0 mV, 15 mV/s). In 12 of these 40 cells, CPA increased [Ca2+]c and IK,Ca signals. If spontaneous transient outward currents were present, they were suppressed by CPA. CPA reduced the peak L-type Ca2+ channel current apparently through increased Ca2+ inactivation of voltage-gated Ca2+ channels. The current could be restored to control by elevating [Ca2+]o from 2.5 to 5 mM. Under these conditions, CPA increased the ramp-induced [Ca2+]c transients from 105.1 +/- 22 to 162.0 +/- 31 nM (n = 9, P < 0.05). These results suggest that Ca2+ sequestration by the SR can buffer part of the Ca2+ influx during slow depolarizations.

This article has been cited by other articles:

				K.-E. Andersson and A. Arner Urinary Bladder Contraction and Relaxation: Physiology and Pathophysiology Physiol Rev, July 1, 2004; 84(3): 935 - 986. [Abstract] [Full Text] [PDF]

				T. Szado, M. McLarnon, X. Wang, and C. van Breemen Role of sarcoplasmic reticulum in regulation of tonic contraction of rabbit basilar artery Am J Physiol Heart Circ Physiol, October 1, 2001; 281(4): H1481 - H1489. [Abstract] [Full Text] [PDF]

				A. Guia, X. Wan, M. Courtemanche, and N. Leblanc Local Ca2+ Entry Through L-Type Ca2+ Channels Activates Ca2+-Dependent K+ Channels in Rabbit Coronary Myocytes Circ. Res., May 14, 1999; 84(9): 1032 - 1042. [Abstract] [Full Text] [PDF]

				R. L. Wardle and R. A. Murphy Minor role of a Ca2+-depleted sarcoplasmic reticulum in heterologous desensitization of smooth muscle to K+ Am J Physiol Cell Physiol, October 1, 1998; 275(4): C1095 - C1103. [Abstract] [Full Text] [PDF]

				S. J. Kim, S. C. Ahn, J. K. Kim, Y. C. Kim, I. So, and K. W. Kim Changes in intracellular Ca2+ concentration induced by L-type Ca2+ channel current in guinea pig gastric myocytes Am J Physiol Cell Physiol, December 1, 1997; 273(6): C1947 - C1956. [Abstract] [Full Text] [PDF]

				E. R. M. Flynn, K. N. Bradley, T. C. Muir, and J. G. McCarron Functionally Separate Intracellular Ca2+ Stores in Smooth Muscle J. Biol. Chem., September 21, 2001; 276(39): 36411 - 36418. [Abstract] [Full Text] [PDF]