We have previously shown that Na⁺-Ca²⁺ exchange current (I_Na/Ca) density is increased (Chorvatova et al. 2001) and that L-type Ca²⁺ current (I_Ca) remains unchanged during catecholamine-induced hypertrophy (Meszaros et al. 1997). The present study investigated the effects of hypertrophy on Ca²⁺ release and Ca²⁺ content of the SR to determine the consequences of increased I_Na/Ca.

Hypertrophy was produced by daily injections of isoprenaline (5.0 mg kg^-1; I.P.) for 7 days. Rats were stunned and killed by cervical dislocation (Schedule 1; Scientific Procedures Act, 1986). The hearts were isolated and digested by protease and collagenase treatment to isolate myocytes. Cells were loaded with fura-2 AM (5 mmol l^-1; 15 min) and field stimulated (0.5 Hz, 2 ms pulse width). Fluorescence ratio (F₃₄₀/F₃₈₀) was converted to [Ca²⁺]_i by in vivo calibration. Ca²⁺ content of the SR was assessed by rapid application of 10 mmol l^-1 caffeine. Composition of the extracellular solution was (mmol l^-1): NaCl, 140; KCl, 5.4; MgCl₂, 1.0; CaCl₂, 1.0; glucose, 10.0 and Hepes, 10.0; pH 7.35 with NaOH. Experiments were conducted at 35 °C. Data are shown as means ± S.E.M. and analysed by Student’s t test for unpaired data.

Membrane capacitance, determined by patch clamp, was increased by 18 % in hypertrophied myocytes (151.5 ± 6.8 pF, n = 32 in control vs. 178.8 ± 8.8 pF, n = 49 hypertrophy, P < 0.05). Peak amplitude of the Ca²⁺ transients was 457 ± 23 nmol l^-1 (n = 24) in control and 685 ± 52 nmol l^-1 (n = 18) in hypertrophied myocytes, representing an increase of ~50 % (P < 0.05). Resting [Ca²⁺]_i was unchanged in the two groups of cells (231 ± 20 in control vs. 218 ± 15 nmol l^-1 hypertrophy). The time constant (t) for the decline of the Ca²⁺ transients was also not significantly different between the two groups (204 ± 9 ms in control and 187 ± 10 ms in hypertrophy). In experiments where caffeine was used to estimate SR Ca²⁺ content, integrals of the Ca²⁺ transients elicited by 10 mmol l^-1 caffeine were increased in hypertrophied myocytes (control, 738.7 ± 27.9 nmol l^-1 ms, n = 24 vs. hypertrophy, 947.8 ± 65.3 nmol l^-1 ms, n = 16, P < 0.05). However, increased SR Ca²⁺ content, also determined by measuring caffeine-induced sarcolemmal current, was not observed when myocytes were held and stimulated from -40 mV.

Data show that SR Ca²⁺ release and Ca²⁺ content were increased in myocytes from hypertrophied hearts, despite an increase in I_Na/Ca density and unchanged I_Ca. The observed increase in SR Ca²⁺ content may therefore result from increased Ca²⁺ entry via reverse Na⁺-Ca²⁺ exchange, and enhance Ca²⁺-induced Ca²⁺ release from the SR.

All procedures accord with current UK legislation.