Purinoceptors in the rat heart

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Abstract
1 The effects of an intracoronary bolus of adenosine triphosphate (ATP), α,β-methylene ATP (APCPP), β,γ-methylene ATP (APPCP), adenosine diphosphate (ADP), adenosine monophosphate (AMP) and adenosine on coronary tone and ventricular myocardial contraction were investigated in the perfused rat heart. 2 Adenine nucleotides, given by bolus injection were negatively inotropic in amounts >3 × 10−7 mol. The potency order was ATP > ADP > AMP. Adenosine (< 1 × 10−5 mol) had no effect on ventricular myocardial contraction. 3 Adenine nucleotides and adenosine (1 × 10−10–1 × 10−7 mol) reduced coronary tone. The potency order was ATP > ADP > AMP = adenosine. The ATP analogue APPCP was less active than ATP at reducing coronary tone, and APCPP had no vasodilator effect. This suggests the presence of a P2-purinoceptor, subclass P2Y, which mediates vasodilatation. 4 ATP and ADP increased the concentration of prostacyclin (measured as 6-keto prostaglandin F1α) in the perfusate, but only after injection of > 3 × 10−7 mol, suggesting that the vasodilator responses to ATP and ADP were not mediated by prostacyclin. AMP and adenosine had no effect, even at 1 × 10−5mol. 5 At a dose of 3 × 10−9 mol, approximately 40% of ATP and 70% of ADP was converted to AMP and adenosine whilst passing through the heart. The amounts of AMP and adenosine formed, however, were insufficient to account for the vasodilator effects of ATP and ADP. 6 Vasodilatation mediated by AMP and adenosine was inhibited by an infusion of 8-phenyltheophyllin (8-PT; 2 × 10−5 m) indicating interaction with a P1-purinoceptor. Vasodilatation induced by ATP (at doses at which AMP and adenosine had no action) was also depressed by 8-PT indicating either an action of ATP on P1-purinoceptors, or an effect of 8-PT on P2Y receptors. 7 Vasodilatation induced by AMP was unaltered during an infusion of α,β-methylene ADP (2 × 10−6 m, which inhibited breakdown of AMP to adenosine by 54.2 ± 1.5%, n = 4). This suggests that AMP acted directly, and it did not require conversion to adenosine to induce vasodilatation. 8 The ATP analogues APCPP (1 × 10−9 −1 × 10−8 mol) and APPCP (1 × 10−8−1 × 10−7 mol) increased coronary tone, as did high doses (1 × 10−5 mol) of ATP and ADP, indicating the presence of an additional P2-purinoceptor, subclass P2X, mediating vasoconstriction.