SURFACE-VOLUME RELATIONSHIP IN CARDIAC MYOCYTES STUDIED WITH CONFOCALMICROSCOPY AND MEMBRANE CAPACITANCE MEASUREMENTS - SPECIES-DEPENDENCEAND DEVELOPMENTAL EFFECTS

The quantitative analysis of the contribution of ion fluxes through me mbrane channels to changes of intracellular ion concentrations would b enefit from the exact knowledge of the cell volume. It would allow dir ect correlation of ionic current measurements with simultaneous measur ements of ion concentrations in individual cells. Because of various l imitations of conventional light microscopy a simple method for accura te cell volume determination is lacking. We have combined the optical sectioning capabilities of fluorescence laser scanning confocal micros copy and the whole-cell patch-clamp technique to study the correlation between cell volume and membrane capacitance. Single cardiac myocytes loaded with the fluorescent dye calcein were optically sectioned to p roduce a series of confocal images. The volume of cardiac myocytes of three different mammalian species was determined by three-dimensional volume rendering of the confocal images. The calculated cell volumes w ere 30.4 +/- 7.3 pl (mean +/- SD) in rabbits (n = 28), 30.9 +/- 9.0 pl in ferrets (n = 23), and 34.4 +/- 7.0 pl in rats (n = 21), respective ly. There was a positive linear correlation between membrane capacitan ce and cell volume in each animal species. The capacitance-volume rati os were significantly different among species (4.58 +/- 0.45 pF/pl in rabbit, 5.39 +/- 0.57 pF/pl in ferret, and 8.44 +/- 1.35 pF/pl in rat) . Furthermore, the capacitance-volume ratio was dependent on the devel opmental stage (8.88 +/- 1.14 pF/pl in 6-month-old rats versus 6.76 +/ - 0.62 pF/pl in 3-month-old rats). The data suggest that the ratio of surface area:volume of cardiac myocytes undergoes significant developm ental changes and differs among mammalian species. We further establis hed that the easily measurable parameters of cell membrane capacitance or the product of cell length and width provide reliable but species- dependent estimates for the volume of individual cells.