P2-PURINOCEPTOR, BUT NOT P1-PURINOCEPTOR MEDIATE FORMATION OF 1,4,5-INOSITOL TRISPHOSPHATE AND ITS METABOLITES VIA A PERTUSSIS TOXIN-INSENSITIVE PATHWAY IN THE RAT RENAL-CORTEX

1. The adenosine receptor (P1-purinoceptor) agonists N6-cyclopentyladenosine and N-5'-ethylcarboxamidoadenosine at concentrations up to 10 μmol l-1 affected neither basal, nor noradrenaline- and angiotensin II-stimulated formation of inositol-1-phosphate, inositol-1,4-bisphosphate, and inositol-1,4,5-trisphosphate in slices of rat renal cortex. 2. In contrast, adenine nucleotides (P2-purinoceptor agonists) markedly stimulated inositol phosphate formation. The observed rank order of potency adenosine-5'-O-(2-thiodiphosphate) (EC50 39 μmol l-1) > adenosine-5'-O-(3-thiotriphosphate) (587)≥ 5'-adenylylimidodiphosphate (App(NH)p, 899) > adenylyl-(β,γ-methylene)-diphosphonate (4,181) was consistent with the interaction of the compounds with the P(2γ)-subtype of P2-purinoceptors. AMP and the ADP analogue (α,β-methylene)-adenosine-5'-diphosphate were ineffective. ATP and ADP (≤ 10 mmol l-1) did not produce a consistent increase, owing to their hydrolytic degradation in the incubation medium. 3. Whereas the inositol phosphate response to App(NH)p was linear only up to 5 min incubation, the time-dependent stimulation of noradrenaline declined at a slower rate. Following pre-exposure of the renal cortical slices to App(NH)p, renewed addition of App(NH)p caused no further enhancement in the accumulation of inositol phosphates, whilst noradrenaline was still capable of eliciting a response. This suggests that the apparent loss of responsiveness to App(NH)p is not due to substrate depletion or enzymatic inactivation, but most likely attributable to homologous desensitization of the purinoceptor. 4. Pretreatment of the animals with pertussis toxin caused a substantial reduction of functional G(i)-protein, as indicated by the lack of [32-P]-NAD incorporation in a membrane preparation of the renal cortex. Nevertheless, the increase in inositol phosphate formation induced by noradrenaline, angiotensin II, and App(NH)p was not significantly impaired. 5. We conclude that P(2γ)-purinoceptors are present in the renal cortex; these receptors stimulate formation of inositol phosphates via a pertussis toxin-insensitive pathway and undergo homologous desensitization. On the other hand, our results suggest that renal A1-adenosine receptors do not use stimulation of phosphoinositide breakdown as a transmembrane signalling system.