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

Abstract

The effects of strychnine (STR) were investigated on K +-stimulated 45Ca 2+-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 45Ca 2+-uptake. When combined at equimolar concentrations (10 −5 M), STR and nifedipine produced a potent (nM) inhibition (40%) of neuronal 45Ca 2+-uptake. In the spontaneously beating rat atria, STR produced a dose-dependent (10 −7−3 × 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 (EC 50 8 × 10 −6 M). Scatchard analysis revealed that STR increased the affinity (decreased the K d) of [ 3H]BAY K 8644 to a greater degree than that of [ 3H]nitrendipine for dihydropyridine binding sites. STR decreased the K d 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 Ca 2+ and partially inhibited by Na + in a concentration-dependent manner. Glycine (10 −2 M) did not affect the STR enhancement of [ 3H]nitrendipine binding. Extensive rinsing of brain membranes increased (∼two-fold) the affinity of STR for enhancement of [ 3H]nitrendipine binding. The effects of STR on [ 3H]nitrendipine binding both resembled and were unique to those of phencyclidine. These findings suggest that STR interacts specifically with calcium channels in brain and cardiac tissues at a site similar, but not identical to that of phencyclidine and which is distinct from the dihydropyridine calcium channel regulatory site.