Studies on the effects of felodipine on the vascular resistance and reactivity in pentobarbital anesthetized dogs

Effects of 5HT2 receptor antagonists on responses to potassium depolarization in rat isolated aorta

1. In order to determine whether 5HT2 receptor antagonists can modify Ca2+ uptake via voltage-operated Ca2+ channel (VOC) in arterial smooth muscle, a comparative study of the effects of selected Ca2+ uptake blockers and 5HT2 receptor antagonists on K(+)-induced contractions of rat aortic strip was undertaken. 2. The antagonist drugs studied included the Ca2+ uptake blockers verapamil, nifedipine, felodipine, diltiazem and cinnarizine, the 5HT2 receptor antagonists cyproheptadine, ritanserin, mianserin, and ketanserin and the alpha 1-adrenoceptor antagonist prazosin. 3. With the notable exception of prazosin, each of these compounds diminished K(+)-induced aortic responses. 4. The following order of potencies (mean IC50 values in mol/L) was established: felodipine (7.0 x 10(-11)) > nifedipine (4.8 x 10(-9) > verapamil (5.5 x 10(-8)) > cyproheptadine (6.2 x 10(-8)) > diltiazem (4.1 x 10(-7)) > cinnarizine (1.3 x 10(-6)) > ritanserin (1.8 x 10(-6)) > ketanserin (9.0 x 10(-6)) > mianserin (2.0 x 10(-5)). 5. The results suggest that antagonists of 5HT2 receptors can modulate Ca2+ uptake via VOC in rat aorta.

Nisoldipine inhibits adrenergic responses in the hindquarters vascular bed of the cat

The effects of the calcium entry blocking agent nisoldipine on adrenergic vasoconstrictor responses were investigated in the hindquarters vascular bed of the cat under conditions of controlled blood flow. Nisoldipine dilated the hindquarters vascular bed and inhibited vasoconstrictor responses to Bay K 8644, a nifedipine analog which promotes calcium entry. During infusion of nisoldipine, vasoconstrictor responses to sympathetic nerve stimulation, norepinephrine, and tyramine were inhibited in a reversible manner. In addition to blocking responses to nerve-released and exogenous norepinephrine, the calcium entry antagonist decreased responses to methoxamine and BHT 933, alpha 1- and alpha 2-adrenoceptor agonists. Responses to methoxamine were reduced by prazosin, an alpha 1-adrenoceptor antagonist, but not by yohimbine, an alpha 2-adrenoceptor blocking agent, whereas responses to BHT 933 were decreased by yohimbine but not by prazosin. The results of these studies suggest that vasoconstrictor responses to neuronally released and exogenous norepinephrine, as well as to selective alpha 1- and alpha 2-adrenoceptor agonists, are dependent in part on an extracellular source of calcium in resistance vessels of the feline hindquarters vascular bed. The inhibitory effect of nisoldipine on vasoconstrictor responses to neuronally released norepinephrine may be important in the antihypertensive actions of calcium entry blocking agents.