Ca²⁺ Sparks Triggered by Patch Depolarization in Rat Heart Cells

From the Departments of Medicine (S.R.S., L.I., W.G.W., C.W.B.) and Physiology (W.G.W., C.W.B.), University of Maryland School of Medicine, Baltimore, Md.

Abstract—The goal of this study was to examine the relationship between Ca²⁺ entry through L-type Ca²⁺ channels and local [Ca²⁺]_i transients (Ca²⁺ sparks) in single rat cardiac ventricular cells. L-type Ca²⁺ channels were activated by depolarization of cell-attached membrane patches, and [Ca²⁺]_i was measured simultaneously as fluo 3 fluorescence using laser scanning confocal microscopy. Patch depolarization with Ca²⁺ as the charge carrier (10 or 110 mmol · L^–1) significantly increased the probability of the occurrence of Ca²⁺ sparks (Ca²⁺ spark rate) only in the volume of cytoplasm located immediately beneath the membrane patch (basal Ca²⁺ spark rate, 119 Ca²⁺ sparks · cell^-1 · s^-1; patch depolarization Ca²⁺ spark rate, 610 Ca²⁺ sparks · cell^-1 · s^-1; P<.005). With Ba²⁺ in the pipette solution (10 mmol · L^-1), patch depolarization was not associated with an increased Ca²⁺ spark rate at the position of the pipette or at any other sites distant from the pipette. Therefore, Ca²⁺ entry and not voltage per se was a necessary event for the occurrence of Ca²⁺ sparks. Under identical experimental conditions, patch depolarization experiments opened single L-type Ca²⁺ channels with a single-channel conductance of 19 pS with Ba²⁺ as the charge carrier. Although single-channel openings could not be resolved when Ca²⁺ was the charge carrier, ensemble averages yielded an inward current of up to 0.75 pA. The results suggest that voltage-activated Ca²⁺ entry through one or a small number of L type Ca²⁺ channels triggers the release of Ca²⁺ only from the sarcoplasmic reticulum in direct proximity to those L-type Ca²⁺ channels. The relatively low probability of triggering Ca²⁺ sparks may have resulted from some alteration of excitation-contraction coupling associated with the technique of the cell-attached patch clamp.

Key Words: heart • excitation-contraction coupling • Ca²⁺ spark • single L-type Ca²⁺ channel

This article has been cited by other articles:

				J. A. Copello, A. V. Zima, P. L. Diaz-Sylvester, M. Fill, and L. A. Blatter Ca2+ entry-independent effects of L-type Ca2+ channel modulators on Ca2+ sparks in ventricular myocytes Am J Physiol Cell Physiol, June 1, 2007; 292(6): C2129 - C2140. [Abstract] [Full Text] [PDF]

				S.-Q. Wang, C. Wei, G. Zhao, D. X.P. Brochet, J. Shen, L.-S. Song, W. Wang, D. Yang, and H. Cheng Imaging Microdomain Ca2+ in Muscle Cells Circ. Res., April 30, 2004; 94(8): 1011 - 1022. [Abstract] [Full Text] [PDF]

				C. V. Remillard, W.-M. Zhang, L. A. Shimoda, and J. S. K. Sham Physiological properties and functions of Ca2+ sparks in rat intrapulmonary arterial smooth muscle cells Am J Physiol Lung Cell Mol Physiol, August 1, 2002; 283(2): L433 - L444. [Abstract] [Full Text] [PDF]

				M. Lohn, M. Furstenau, V. Sagach, M. Elger, W. Schulze, F. C. Luft, H. Haller, and M. Gollasch Ignition of Calcium Sparks in Arterial and Cardiac Muscle Through Caveolae Circ. Res., November 24, 2000; 87(11): 1034 - 1039. [Abstract] [Full Text] [PDF]

				W. G. Wier and C. W. Balke Ca2+ Release Mechanisms, Ca2+ Sparks, and Local Control of Excitation-Contraction Coupling in Normal Heart Muscle Circ. Res., October 29, 1999; 85(9): 770 - 776. [Full Text] [PDF]