Involvement of G-protein βγ subunits on the influence of inhibitory α 2-autoreceptors on the angiotensin AT 1-receptor modulation of noradrenaline release in the rat vas deferens

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The influence of α 2-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 AT 1-receptor antagonist losartan (0.3–1 μM), at concentrations suggesting the involvement of the AT 1B subtype. The effect of angiotensin II was also attenuated by inhibition of phospholipase C (PLC) and protein kinase C (PKC) indicating that prejunctional AT 1-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 α 2-autoinhibition, observed when stimulation consisted of 100 pulses at 8 Hz, than under poor α 2-autoinhibition conditions, only up to 14%, observed when α 2-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 α 2-autoinhibition conditions. Inhibition of G i/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 AT 1-receptors coupled to the PLC–PKC pathway enhances noradrenaline release, an effect that is markedly favoured by an ongoing activation of α 2-autoreceptors. Interaction between α 2-adrenoceptors and AT 1-receptors seems to involve the βγ subunits released from the G i/o-proteins coupled to α 2-adrenoceptors and protein kinase C activated by AT 1-receptors.