Involvement of G-protein βγ subunits on the influence of inhibitory α2-autoreceptors on the angiotensin AT1-receptor modulation of noradrenaline release in the rat vas deferens

The influence of α2-autoreceptors on the facilitation of [3H]-noradrenaline release mediated by angiotensin II was studied in prostatic portions of rat vas deferens preincubated with [3H]-noradrenaline. Angiotensin II enhanced tritium overflow evoked by trains of 100 pulses at 8 Hz, an effect that was attenuated by the AT1-receptor antagonist losartan (0.3–1 μM), at concentrations suggesting the involvement of the AT1B subtype. The effect of angiotensin II was also attenuated by inhibition of phospholipase C (PLC) and protein kinase C (PKC) indicating that prejunctional AT1-receptors are coupled to the PLC–PKC pathway.Angiotensin II (0.3–100 nM) enhanced tritium overflow more markedly, up to 64%, under conditions that favor α2-autoinhibition, observed when stimulation consisted of 100 pulses at 8 Hz, than under poor α2-autoinhibition conditions, only up to 14%, observed when α2-adrenoceptors were blocked with yohimbine (1 μM) or when stimulation consisted of 20 pulses at 50 Hz. Activation of PKC with 12-myristate 13-acetate (PMA, 0.1–3 μM) also enhanced tritium overflow more markedly under strong α2-autoinhibition conditions. Inhibition of Gi/o-proteins with pertussis toxin (8 μg/ml) or blockade of Gβγ subunits with the anti-βγ peptide MPS-Phos (30 μM) attenuated the effects of angiotensin II and PMA.The results indicate that activation of AT1-receptors coupled to the PLC–PKC pathway enhances noradrenaline release, an effect that is markedly favoured by an ongoing activation of α2-autoreceptors. Interaction between α2-adrenoceptors and AT1-receptors seems to involve the βγ subunits released from the Gi/o-proteins coupled to α2-adrenoceptors and protein kinase C activated by AT1-receptors.