In cardiac hypertrophy and failure there is an over-expression of the Na⁺-Ca²⁺ exchanger but its contribution to E-C coupling in these conditions remains unclear (Hasenfuss et al. 1999). We have examined aspects of Ca²⁺ regulation in hypertrophied mouse hearts in which the Na⁺-Ca²⁺ exchanger is either normally expressed (wild-type, WT) or over-expressed (NCX).

Hypertrophy was induced by a constant infusion (14 days at 1 µg min^-1 kg^-1) of angiotensin II via osmotic pumps implanted subcutaneously under isoflurane anaesthesia. Control mice were pumped with saline. Cardiac myocytes were enzymatically dissociated from mouse hearts following cervical dislocation (Terracciano et al. 1998). Cells were loaded with fluo-4 AM and all experiments carried out at 37 °C. Confocal microscopy was used to detect Ca²⁺ sparks. Sarcoplasmic reticulum (SR) Ca²⁺ contents were measured using rapid caffeine application and are expressed as µM l^-1 (non-mitochondrial volume). Data are expressed as means ± S.E.M. with statistical differences assessed using a one-way ANOVA with a Bonferroni post-test. P < 0.05 was considered significant.

Angiotensin II infusion (A) led to a significant increase in cell capacitance for both WT and NCX myocytes compared with saline (S) (capacitance (pF) = WT-S: 165 ± 11, n = 27; NCX-S: 166 ± 13, n = 18; WT-A: 220 ± 7, n = 28; NCX-A: 229 ± 12, n = 33; P < 0.05). This result suggests equivalent cellular hypertrophy was induced in WT and NCX.

The frequency of Ca²⁺ sparks at rest in WT-S myocytes was less than in NCX-S myocytes while there was no apparent difference between the WT-A and NCX-A groups (frequency (Hz) = WT-S: 0.13 ± 0.03, n = 43; NCX-S: 0.50 ± 0.13, n = 35, P < 0.05; WT-A: 0.06 ± 0.02, n = 41; NCX-A: 0.24 ± 0.08, n = 35). The SR Ca²⁺ contents in WT myocytes were less than that of NCX myocytes (SR contents (µM l^-1) = WT-S: 57.5 ± 5.7, n = 13; WT-A: 61.2 ± 4.9, n = 14; NCX-S: 92.5 ± 8.8, n = 5; NCX-A: 88.9 ± 8.8, n = 11, P < 0.05). The proportion of myocytes that failed to show any Ca²⁺ sparks for the duration of the linescan increased from 0.55 (n = 78) in WT-S to 0.76 (n = 71) in WT-A. In NCX myocytes the proportion of cells failing to show Ca²⁺ sparks increased from 0.44 (n = 55) in NCX-S to 0.62 (n = 35) in NCX-A.

In conclusion we found that NCX mice had increased frequency of Ca²⁺ sparks and SR Ca²⁺ content. The effect of exposure to angiotensin II as a hypertrophic stimulus did not alter the SR Ca²⁺ contents in these myocytes.This work was supported by The Wellcome Trust.

Hasenfuss, G., Schillinger, W., Lehnart, S.E., Preuss, M., Pieske, B., Maier, L.S., Prestle, J., Minami, K. & Just, H. (1999). Circulation 99, 641-648.

Terracciano, C.M.N., DeSouza, A.I., Philipson, K.D. & MacLeod, K.T. (1998). J. Physiol. 512, 651-667. abstract