Clinical studies indicate that testosterone replacement therapy is beneficial to the cardiovascular system, and reduced levels of testosterone are associated with conditions such as coronary heart disease (English et al. 2000a,b). In addition, in vitro studies have shown that testosterone acts as a vasodilator in vessels from various vascular beds (reviewed by Jones et al. 2003). Since Ca²⁺ influx through L-type Ca²⁺ channels is a major determinant of vascular tone, we have investigated whether testosterone may act via blockade of L-type Ca²⁺ channels. To do this, we have employed whole-cell patch clamp recordings (using 20 mM Ba²⁺ as the charge carrier) from HEK 293 cells stably expressing the α_1C subunit of a human L-type Ca²⁺ channel (hHT splice variant) of the cardiovascular system (see Fearon et al. (2000) for further experimental details).

Bath application of testosterone (1-10 000 nM) caused a concentration-dependent inhibition of L-type Ca²⁺ current amplitudes with an estimated IC₅₀ of 61.0 nM (n = 4-9 cells at each of 5 concentrations tested). At the lowest concentration examined (1 nM), currents were reduced by 25.4 ± 2.6 % (mean ± S.E.M., n = 4 cells). This inhibition did not achieve statistical significance (Student’s paired t test), but the effects of testosterone were partially reversible. At all higher concentrations, the inhibitory effects of testosterone were always irreversible for at least 10 min of recording. At 1000 nM, testosterone inhibited currents maximally by 82.7 ± 3.8 % (P < 0.001, n = 9). Current-voltage relationships indicated that testosterone caused a similar degree of inhibition at all activating test potentials, suggesting that its inhibitory effect was voltage-independent. No significant effects of testosterone were seen on channel kinetics.

Our results suggest that testosterone – at physiologically relevant concentrations – acts as an inhibitor of human L-type Ca²⁺ channels via a direct (i.e. non-genomic) mechanism, raising the intriguing possibility that it may act as an endogenous dihydropyridine antagonist. If true, this would in turn suggest testosterone binds to the α subunit of the channel, since no auxiliary subunits were co-expressed in this study. Such an action may account for its beneficial cardiovascular effects in vivo.

This work was supported by the British Heart Foundation.