Inhibition of Signal Ca2+ in Dog Coronary Arterial Vascular Muscle Cells by Ro 40–5967

The effects of Ro 40-5967, a chemically novel Ca2+ channel antagonist, were examined on K(+)- and norepinephrine (NE)-stimulated intracellular free Ca2+ concentrations in dog coronary artery vascular muscle cells (VMC). Intracellular Ca2+ activity increases stimulated by NE or K+ were measured by shuttered, low-intensity illumination with the fluorescent Ca2+ indicator fluo3 in single myovascular cells < or = 2 days after isolation. Pretreatment of VMC for 5 min with 10 microM Ro 40-5967 significantly reduced intracellular Ca2+ activity increase on exposure to 100 mM K+ in both subsarcolemmal (SSL) and central regions during K+ depolarization, similar to actions of diltiazem. More impressively, Ro 40-5967 (but not diltiazem) also reduced intracellular Ca2+ activity during stimulation with 100 nM NE to basal level at both peripheral and central regions. Prevention of Ca2+ increases, especially those caused by release from central Ca2+ stores, during NE stimulation is an unusual property for a Ca2+ antagonist, as shown by the contrast with diltiazem, which did not significantly attenuate Ca2+ increases during NE stimulation. Another Ro 40-5967 feature was vascular selectivity; i.e., Ro 40-5967 had only a minor effect on contraction frequency of rat myocardial cells while significantly inhibiting spontaneous contractions of rat spontaneously active VMC. In comparison, diltiazem and verapamil potently inhibited both vascular and cardiac muscle spontaneous contractions. The decidedly stronger actions of Ro 40-5967 as compared with those of benzothiazepine or phenylalkylamine on NE-stimulated Ca2+ increases, together with less action on cardiac pacemakers, are differences that appear to distinguish Ro 40-5967 from other Ca2+ antagonists.