Murine ventricular L‐type Ca2+ current is enhanced by zinterol viaβ1‐adrenoceptors, and is reduced in TG4 mice overexpressing the human β2‐adrenoceptor

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Abstract
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. 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. 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. 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. British Journal of Pharmacology (2001) 133, 73–82; doi:10.1038/sj.bjp.0704045