Effects of bromocriptine on catecholamine receptors mediating cardiovascular responses in the pithed rat

The interaction of bromocriptine with several catecholamine receptors that control the sympathetic responses at cardiac and vascular level has been studied in pithed adrenalectomized and vagotomized normotensive rats. Bromocriptine (30 and 100 micrograms/kg) inhibited the stimulation-induced pressor responses in the pithed rat without modifying the pressor responses induced by noradrenaline. Sulpiride (0.3 mg/kg) abolished the effects of bromocriptine (30 micrograms/kg) but only partially prevented the effects of bromocriptine (100 micrograms/kg) on the stimulation-induced pressor responses. Yohimbine (0.3 mg/kg) partially antagonised the inhibitory effect of bromocriptine on stimulation-induced pressor responses. Combination of yohimbine and sulpiride abolished attenuation of the stimulation-induced pressor responses by bromocriptine (100 micrograms/kg). Bromocriptine (0.3 and 1 mg/kg) shifted to the right the frequency-response curve of increases in heart rate. This effect was prevented by yohimbine (0.3 mg/kg) but not by sulpiride (0.3 mg/kg). The same doses of bromocriptine were ineffective on heart rate increases induced by noradrenaline. Bromocriptine (0.3 and 1 mg/kg) shifted to the right the increases in diastolic blood pressure induced by methoxamine without modifying those induced by xylazine and noradrenaline. These results suggest that bromocriptine acts on the peripheral sympathetic nervous system of the pithed rat as an agonist of presynaptic dopamine receptors and alpha 2-adrenoreceptors and as an antagonist of postsynaptic alpha 1-adrenoreceptors.