L-type calcium current and contractility in ventricular myocytes from miceoverexpressing the cardiac beta(2)-adrenoceptor

Objectives: The reported increase in basal activity of hearts from transgen ic mice (TG4) overexpressing the human beta(2)-adrenoceptor (beta(2)-AR) wa s explained by spontaneously active beta(2)-ARs that stimulate the beta-adr energic cascade in the absence of an agonist. In order to examine altered m yocardial function on a cellular level, we have investigated L-type calcium current (I-Ca,I-L) and cell shortening in ventricular myocytes from TG4 he arts. Myocytes from Littermates (LM) and wild type animals (WT) served as c ontrols. Methods: Cardiac P-AR density was measured by [I-125]-iodocyanopin dolol binding to ventricular membranes. I-Ca,I-L was assessed by standard w hole-cell voltage clamp technique. Contractility was measured as cell short ening in ventricular myocytes and as force of contraction in electrically s timulated left atria. Results: Overexpression of beta(2)-ARs was confirmed by an almost 400-fold increase in beta-AR density. The beta(1):beta(2)-AR r atio in WT mice was 71:29. Myocytes from TG4 and LM mice were similar in si ze as judged by membrane capacitance and two dimensional cell area. I-Ca,I- L amplitude was significantly lower in TG4 than in LM myocytes (with 2 mM [ Ca2+](o) -4.82+/-0.48 vs. -6.56+/-0.38 pA/pF, respectively). In TG4 myocyte s, the I-Ca,I-L response to isoproterenol (1 mu M) was almost abolished. Ce ll shortening was not different in physiological [Ca2+](o), but smaller in maximum [Ca2+](o) when comparing TG4 to control myocytes. Basal force of co ntraction in left atria did not differ between TG4 and LM at any age invest igated. In TG4 left atria the inotropic response to isoproterenol was also absent, whereas responses to high [Ca2+](o) or dibutyryl-cAMP (1 mM) were p resent but reduced. The rate of spontaneous beating of right atria was elev ated in TG4 mice. Conclusions: Since only spontaneous beating rate but neit her basal I-Ca,I-L amplitude nor basal contractile activity were elevated, our data fail to reveal evidence for spontaneously active, stimulating beta (2)-ARs in left atrium and ventricle. A contractile deficit unrelated to th e P-adrenoceptor pathway is evident in TG4 myocytes and left atria. (C) 199 9 Elsevier Science B.V. All rights reserved.