Ca²⁺ wave characteristics in cardiac muscle have been studied over a range of cellular Ca²⁺ values by a number of groups. In one study, the results suggest that Ca²⁺ wave properties are diverse and modulated by the Ca²⁺ loading state (Kaneko et al. 2000). In another, only Ca²⁺ wave frequency increases as cellular Ca²⁺ load is increased (Diaz et al. 1997).

To study the characteristics of Ca²⁺ waves in detail, rabbit ventricular myocytes were isolated from hearts removed after humanely killing the animal with an overdose of sodium pentobarbitone (100 mg kg^-1 I.V.). Spontaneous SR Ca²⁺ release was evoked in isolated myocytes permeabilised with β-escin (0.01 mg) and perfused with a mock intracellular solution containing [Ca²⁺] at 300 nM. Ca²⁺ concentration within myocytes was measured using laser-scanning confocal epifluorescence microscopy to excite the Ca²⁺-sensitive dye Fluo-5F pentapotassium salt (10 µM). Using previous estimates of cytoplasmic Ca²⁺ buffering (Hove-Madsen et al. 1993) the Ca²⁺ fluxes underlying Ca²⁺ waves could be calculated from the time course of the Ca²⁺ wave. Total SR content was assessed by rapid application of caffeine (10 mM).

These calculations suggest the SR Ca²⁺ content during spontaneous release (151 ± 11 µmoles l^-1 cell volume, n = 5) is not significantly different from the total Ca²⁺ release during each wave (162 ± 7 µmoles l^-1 cell volume, S.E.M., n = 5) (P > 0.05). As shown in Fig. 1, model calculations based on initial parameters measured at 300 nM, predicted a frequency and amplitude dependence over a range of cytosolic [Ca²⁺] (300-1000 nM) that was similar to that measured experimentally (filled squares). This indicates that a common threshold mechanism explains the dependency of Ca²⁺ wave frequency and amplitude on the mean intracellular Ca²⁺ levels (up to 1 µM).

EPSRC and BHF financially supported this work.