Temporal and spatial properties of cellular Ca flux in trout ventricular myocytes

Shiels, Holly A., and Ed White. Temporal and spatial properties of cellular Ca flux in trout ventricular myocytes. Am J Physiol Regul Integr Comp Physiol 288: R1756–R1766, 2005. First published January 13, 2005; doi:10.1152/ajpregu.00510.2004.—Confocal microscopy was used to investigate the temporal and spatial properties of Ca transients and Ca sparks in ventricular myocytes of the rainbow trout (Oncorhynchus mykiss). Confocal imaging confirmed the absence of T tubules and the long ( 160 m), thin ( 8 m) morphology of trout myocytes. Line scan imaging of Ca transients evoked by electrical stimulation in cells loaded with fluo 4 revealed spatial inhomogeneities in the temporal properties of Ca transients across the width of the myocytes. The Ca wavefront initiated faster, rose faster, and reached larger peak amplitudes in the periphery of the myocyte compared with the center. These differences were exacerbated by stimulation with the L-type Ca channel agonist ( )BAY K 8644 or by sarcoplasmic reticulum (SR) inhibition with ryanodine and thapsigargin. Results reveal that the shape of the trout myocyte allows for rapid diffusion of Ca from the cell periphery to the cell center, with SR Ca release contributing to the cytosolic Ca rise in a time-dependent manner. Spontaneous Ca sparks were exceedingly rare in trout myocytes under control conditions (1 sparking cell from 238 cells examined). This is in marked contrast to the rat where a total of 56 spontaneous Ca sparks were observed in 9 of 11 myocytes examined. Ca sparklike events were observed in a very small number of trout myocytes (15 sparks from 9 of 378 cells examined) after stimulation with either ( )BAY K 8644 or high Ca (6 mM). Reducing temperature to 15°C in intact myocytes or permeabilizing myocytes to adjust intracellular conditions to favor Ca spark detection was without significant effects. Possible reasons for the rarity of Ca sparks in a cardiac myocyte with an active SR are discussed.