Bay K 8644 enhances the outflow of [³H]-noradrenaline and [³H]-DOPEG from isolated rat atria

The effect of Bay K 8644 (a dihydropyridine Ca²⁺-channel activator), was examined on spontaneous and stimulus-evoked release of tritium from isolated rat atria prelabelled with [³H]-noradrenaline. Bay K8644 (3μmol/l) significantly increased atrial rate from 206±7 to 259±9 beats·min^-1 (P<0.05) and also tritium outflow (expressed as fractional rate of loss in min^- × 103) from 6.49±0.35 to 8.61±0.74 (P<0.05). Neither the maximal rate nor the overflow of tritium induced by stimulation of sympathetic nerve terminals was changed by the compound. The increase in basal tritium outflow produced by Bay K 8644 was calcium-dependent. However, it could not be antagonized by nitrendipine. The overflow of tritium induced by Bay K 8644 consisted mainly of 3,4-dihydroxyphenylglycol ([³H]-DOPEG), indicating that the compound produces a leakage from the storage vesicles of sympathetic nerve terminals of the isolated rat atria.