Chronic Administration of Nifedipine Induces Up-regulation of Functional Calcium Channels in Rat Myocardium

Previous studies from our laboratory demonstrated the up-regulation of cardiac dihydropyridine (DHP) receptors in rabbits chronically treated with nifedipine (NIFE). The goal of the present study was to further examine the functionality of this increased number of receptors by analysing different steps of excitation contraction coupling mechanism in adult rats chronically treated with NIFE (a single 10-mg oral dose/kg/day for 28 days). Ca2+channel density was assessed by specific binding at the DHP receptors with [methyl-3H]PN 200-110 in rat ventricular membranes. Chronic NIFE treatment produced up-regulation of Ca2+channels, being the maximal binding capacities 222±19 fmol/mg protein (n=14) and 310±21 fmol/mg protein (n=11) in untreated and treated animals, respectively (P<0.05). The functional consequences of this up-regulation of Ca2+channels were determined in isolated ventricular myocytes by measuring -type Ca2+currents (ICa) with the whole-cell configuration of patch-clamp technique and by intracellular Ca2+(Ca2+i) transients estimated by the Indo-1/AM fluorescence ratio (410/482) simultaneously monitored with cell shortening. Peak ICadensity recorded at 0 mV was 32% greater in myocytes isolated from the treated group than in those obtained from the untreated group (−10.43±0.73 pA/pF (n=13) vs−7.10±0.59 pA/pF (n=12) P<0.05). Ca2+itransient amplitude and cell shortening, explored at 1 and 2 m extracellular calcium ([Ca]0) were significantly higher in ventricular myocytes obtained fom NIFE-treated rats than in myocytes isolated from untreated animals. At 2 m [Ca]0, the values of Ca2+itransient and shortening were 460±61 n and 11±1% of resting length (L0) in myocytes from treated rats (n=9) and 212±22 n and 5.3±0.5% of L0in myocytes from control rats (n=6, P<0.05). The results demonstrate an up-regulation of functionally-active cardiac Ca2+channels after NIFE treatment, and offer a possible explanation for a “withdrawal effect” at myocardial level after the suppression of the treatment with this drug.