Chronic verapamil treatment remodels I_Ca,L in mouse ventricle

In this study we tested the hypothesis that ventricular homeostasis of L-type Ca²⁺ current (I_Ca,L) minimally involves regulation of the main pore-forming α-subunit (Ca_V1.2) and auxiliary proteins that serve as positive or negative regulators of I_Ca,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 I_Ca,L. Surprisingly, 24- to 48-h Ver increased Ca_V1.2 mRNA and protein and I_Ca,L current (Ver 11 ± 1pA/pF vs. control 7 ± 0.5pA/pF; P < 0.01). I _Ca,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_V1.2- and Ca_Vβ₂ 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 I_Ca,L density and diminished responsiveness to acute 1 μM 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,L, with the exception that it prevented the increase in Rem seen with Iso treatment. Long-term Ca²⁺ channel blockade induces an increase of Ca _V1.2 mRNA and protein and significantly increases I_Ca,L.