Chronic administration of nifedipine induces up-regulation of functional calcium channels in rat myocardium

Previous studies from our laboratory demonstrated the up-regulation of card iac dihydropyridine (DHP) receptors in rabbits chronically treated with nif edipine (NIFE). The goal of the present study was to further examine the fu nctionality of this increased number of receptors by analysing different st eps of excitation contraction coupling mechanism in adult rats chronically treated with NIFE (a single 10-mg oral dose/kg/day for 28 days). Ca2+ chann el density was assessed by specific binding at the DHP receptors with [meth yl-H-3]PN 200-110 in rat ventricular membranes. Chronic NIFE treatment prod uced up-regulation of Ca2+ channels, being the maximal binding capacities 2 22 +/-19 fmol/mg protein (n=14) and 310+/-21fmol/mg protein (n=11) in untre ated and treated animals, respectively (P<0.05). The functional consequence s of this up-regulation of Ca-+(2) channels were determined in isolated ven tricular myocytes by measuring L-type Ca2+ currents (I-Ca) with the whole-c ell configuration of patch-clamp technique and by intracellular Ca2+ (Ca-i( 2+)) transients estimated by the Indo-1/AM fluorescence ratio (410/482) sim ultaneously monitored with cell shortening. Peak I-Ca density recorded at 0 mV was 32% greater in myocytes isolated from the treated group than in tho se obtained from the untreated group (-10.43 +/- 0.73 pA/pF (n=13) vs - 7.1 0 +/- 0.59 pA/pF (n = 12); P<0.05). Ca-1(2+), transient amplitude and cell shortening, explored at 1 and 2 mM extracellular calcium ([Ca](o)) were sig nificantly higher in ventricular myocytes obtained fom NIFE-treated rats th an in myocytes isolated from untreated animals. At 2 mM [Ca](o), the values of Ca-1(2+) transient and shortening were 460 +/- 61 nM and 11 +/- 1% of r esting length (L-o) in myocytes from treated rats (n = 9) and 212 +/- 22 nM and 5.3 +/- 0.5% of L-o in myocytes from control rats (n=6, P<0.05). The r esults demonstrate an up-regulation of functionally-active cardiac Ca2+ cha nnels after NIFE treatment, and offer a possible explanation for a "withdra wal effect" at myocardial level after the suppression of the treatment with this drug. (C) 1999 Academic Press.