Murine ventricular L-type Ca2+ current is enhanced by zinterol via β1-adrenoceptors, and is reduced in TG4 mice overexpressing the human β2-adrenoceptor

Abstract

1. The functional coupling of β2-adrenoceptors (β2-ARs) to murine L-type Ca2+ current (ICa(L)) was investigated with two different approaches. The β2-AR signalling cascade was activated either with the β2-AR selective agonist zinterol (myocytes from wild-type mice), or by spontaneously active, unoccupied β2-ARs (myocytes from TG4 mice with 435 fold overexpression of human β2-ARs). Ca2+ and Ba2+ currents were recorded in the whole-cell and cell-attached configuration of the patch-clamp technique, respectively. 2. Zinterol (10 μM) significantly increased ICa(L) amplitude of wild-type myocytes by 19 ± 5%, and this effect was markedly enhanced after inactivation of Gi-proteins with pertussis-toxin (PTX; 76 ± 13% increase). However, the effect of zinterol was entirely mediated by the β1-AR subtype, since it was blocked by the β1-AR selective antagonist CGP 20712A (300 nM). The β2-AR selective antagonist ICI 118,551 (50 nM) did not affect the response of ICa(L) to zinterol. 3. In myocytes with β2-AR overexpression ICa(L) was not stimulated by the activated signalling cascade. On the contrary, ICa(L) was lower in TG4 myocytes and a significant reduction of single-channel activity was identified as a reason for the lower whole-cell ICa(L). The β2-AR inverse agonist ICI 118,551 did not further decrease ICa(L). PTX-treatment increased current amplitude to values found in control myocytes. 4. In conclusion, there is no evidence for β2-AR mediated increases of ICa(L) in wild-type mouse ventricular myocytes. Inactivation of Gi-proteins does not unmask β2-AR responses to zinterol, but augments β1-AR mediated increases of ICa(L). In the mouse model of β2-AR overexpression ICa(L) is reduced due to tonic activation of Gi-proteins.