Chronic verapamil treatment remodels ICa,L in mouse ventricle

In this study we tested the hypothesis that ventricular homeostasis of L-type Ca2+ current (I-Ca,I-L) minimally involves regulation of the main pore-forming alpha-subunit (Ca(V)1.2) and auxiliary proteins that serve as positive or negative regulators of ICa, L. We treated animals for 24 h with verapamil (Ver, 3.6 mg.kg(-1).day(-1)), isoproterenol (Iso, 30 mg.kg(-1).day(-1)), or Iso + Ver via osmotic minipumps. To test for alterations of Ca2+ channel complex components we performed real-time PCR and Western blot analysis on ventricle. In addition, cardiac myocytes (CMs) were dispersed and current was recorded in the whole cell configuration to evaluate ICa, L. Surprisingly, 24- to 48-h Ver increased CaV1.2 mRNA and protein and I-Ca,I-L current (Ver 11 +/- 1pA/pF vs. control 7 +/- 0.5pA/pF; P < 0.01). I-Ca,I-L from CMs in Ver mice showed no change in whole cell capacitance. To examine the in vivo effects of a physiologically relevant Ca2+ channel agonist, we treated mice with Iso. Twenty-four-hour Iso infusion increased heart rate; Ca(V)1.2- and Ca-V beta(2) mRNA levels were constant, but the Ca2+ channel subunit mRNA Rem was increased twofold. Cells isolated from 24- h Iso hearts showed no change in basal ICa, L density and diminished responsiveness to acute 1 mu M Iso. To further examine the homeostatic regulation of the Ca2+ channel, we treated animals for 24 h with Iso + Ver. The influence of Iso + Ver was similar that of to Iso alone on Ca2+ channel mRNAs and I-Ca,I-L, with the exception that it prevented the increase in Rem seen with Iso treatment. Long-term Ca2+ channel blockade induces an increase of CaV1.2 mRNA and protein and significantly increases I-Ca,I-L.