Modulation of L-type Ca2+ channels by beta3-adrenoceptor activation and the involvement of nitric oxide

Both beta1- and beta2-adrenoceptors (AR) in cardiac tissues are responsible for the excitatory effect of catecholamines. Recent evidence demonstrated the presence of another subtype of beta-AR (beta3-AR) in cardiac ventricular preparation. Activation of beta3-AR elicited a depressant response on ventricular contraction. The underlying mechanism(s) for the negative inotropism is relatively unknown.

We investigated the effects of beta3-AR activation on basal voltage-dependent Ca2+ channel (I(CaL)) amplitude of the guinea pig enzyme-dissociated single ventricular myocytes using amphotericin B (200 microg/mL) perforated-patch whole-cell patch-clamp techniques (approximately 22 degrees C).

Application of (-)-isoprenaline ((-)-ISO) (100 nM, a nonselective beta-AR agonist) increased the basal I(CaL) amplitude (approximately 210% of control) (n = 8). However, in the presence of nadolol (1 degreesM, a beta1-/beta2-AR antagonist), the stimulatory effect of (-)-ISO on I(CaL )was abolished and a slowly developed inhibition of the basal I(CaL) was recorded (approximately 80% of control) (n = 9). A smaller degree of inhibition was observed with BRL 37344 (100 nM, a selective beta3-AR agonist) (58% of control) (n = 7). The inhibitory effect of (-)-ISO (with nadolol) and BRL 37344 persisted after washout. Pretreating the ventricular myocytes with L-NAME (0.3 microM, a nitric oxide synthase inhibitor), but not D-NAME (0.3 microM), abolished the inhibitory effect of (-)-ISO and BRL 37344 (n = 7-9).

The results suggest that beta3-ARs are present in ventricular myocytes. Activation of the beta3-AR resulted in an inhibition of the basal I(CaL) amplitude probably due to the formation of nitric oxide.