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


METHODS
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).


RESULTS
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).


CONCLUSIONS
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.
Au
Alice L S
AL
Department of Pharmacology, Faculty of Medicine, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, Peoples Republic of China.
Kwan
Yiu Wa
YW
D000318:Adrenergic beta-Agonists / Q000494:pharmacology*; D000818:Animals; D020746:Calcium Channels, L-Type / Q000378:metabolism*; D003196:Computer Graphics; D004983:Ethanolamines / Q000494:pharmacology; D006168:Guinea Pigs; D007545:Isoproterenol / Q000494:pharmacology*; D032383:Myocytes, Cardiac / Q000378:metabolism; D009569:Nitric Oxide / Q000378:metabolism; D022702:Receptors, Adrenergic, beta-3 / Q000187:drug effects*
D000318:Adrenergic beta-Agonists; D020746:Calcium Channels, L-Type; D004983:Ethanolamines; D022702:Receptors, Adrenergic, beta-3; D009569:Nitric Oxide; C057368:BRL 37344; D007545:Isoproterenol