Differential effects of strychnine on two types of vascular muscle

The effects of strychnine on the regulation of voltage-dependent calcium channels by dihydropyridines in brain and heart

The effects of strychnine (STR) were investigated on K(+)-stimulated 45Ca2(+)-uptake into mouse brain neurons, the contractile activity of spontaneously beating rat atria and on [3H]nitrendipine and [3H]BAY K 8644 binding to dihydropyridine calcium channel antagonist and agonist binding sites on brain and cardiac membranes. STR (10(-6)-10(-4) M) had no effect on neuronal 45Ca2(+)-uptake. When combined at equimolar concentrations (10(-5) M), STR and nifedipine produced a potent (nM) inhibition (40%) of neuronal 45Ca2(+)-uptake. In the spontaneously beating rat atria, STR produced a dose-dependent (10(-7)-3 x 10(-4) M) decrease in chronotropy but did not affect inotropy. STR (10(-4) M) completely inhibited the positive chronotropic, but did not affect the positive inotropic effects of (-)-S-BAY K 8644. [3H]Nitrendipine and [3H]BAY K 8644 binding to brain and cardiac membranes was enhanced by STR in a concentration-dependent manner (EC50 8 X 10(-6) M). Scatchard analysis revealed that STR increased the affinity (decreased the Kd) of [3H]BAY K 8644 to a greater degree than that of [3H]nitrendipine for dihydropyridine binding sites. STR decreased the Kd of [3H]nitrendipine binding by increasing and decreasing the microassociation and microdissociation constants respectively. STR enhanced [3H]nitrendipine binding to the same extent in the cerebral cortex, striatum, hippocampus, cerebellum, brainstem and spinal cord. The enhancement of [3H]nitrendipine binding in brain was completely inhibited by Ca2+ and partially inhibited by Na+ in a concentration-dependent manner. Glycine (10(-2) M) did not affect the STR enhancement of [3H]nitrendipine binding.(ABSTRACT TRUNCATED AT 250 WORDS)

Suppression of strychnine on the chronotropic and inotropic effects in the isolated, blood-perfused canine atrium

In the isolated, blood-perfused dog atrium, strychnine evoked dose-dependent decreases in atrial rate and contractile force, and the negative cardiac effects were not influenced by atropine. Strychnine suppressed dose-dependently the negative chronotropic and inotropic responses to intramural parasympathetic nerve stimulation, but not the response to acetylcholine. These results suggest that strychnine has direct cardiac depressant properties and inhibits the release of acetylcholine from parasympathetic nerve terminals in isolated dog atrium.