Nucleotide coronary vasodilation in guinea pig hearts

Abstract

The role of P1 receptors and P2Y1 receptors in coronary vasodilator responses to adenine nucleotides was examined in the isolated guinea pig heart. Bolus arterial injections of nucleotides were made in hearts perfused at constant pressure. Peak increase in flow was measured before and after addition of purinoceptor antagonists. Both the P1 receptor antagonist 8-(p-sulfophenyl)theophylline and adenosine deaminase inhibited adenosine vasodilation. AMP-induced vasodilation was inhibited by P1 receptor blockade but not by adenosine deaminase or by the selective P2Y1 antagonist N6-methyl-2'-deoxyadenosine 3',5'-bisphosphate (MRS 2179). ADP-induced vasodilation was moderately inhibited by P1 receptor blockade and greatly inhibited by combined P1 and P2Y1 blockade. ATP-induced vasodilation was antagonized by P1 blockade but not by adenosine deaminase. Addition of P2Y1 blockade to P1 blockade shifted the ATP dose-response curve further rightward. It is concluded that in this preparation ATP-induced vasodilation results primarily from AMP stimulation of P1 receptors, with a smaller component from ATP or ADP acting on P2Y1 receptors. ADP-induced vasodilation is largely due to P2Y1 receptors, with a smaller contribution by AMP or adenosine acting via P1 receptors. AMP responses are mediated solely by P1 receptors. Adenosine contributes very little to vasodilation resulting from bolus intracoronary injections of ATP, ADP, or AMP.

Fig. 1.

Flow responses to 3 bolus intracoronary ATP injections from 1 heart. Injection volumes were 25 μl. Concentrations refer to the ATP concentration in the bolus injectate. Responses to 2 injections at intermediate concentrations are not shown. ATP-induced vasodilation results in transient flow increases in this constant-pressure preparation. Peak increase in flow above baseline was used as the response to each injection.

Fig. 2.

Responses to intracoronary adenosine (ADO) injections. A control and an experimental dose-response curve were obtained in each heart. Mean responses from 2 hearts are shown in each panel. Both P1 receptor blockade with 50 μM 8-(p-sulfophenyl) theophylline (8-SPT; A) and addition of 6 U/ml adenosine deaminase (ADA) to the perfusate (B) effectively antagonize adenosine responses and shift the dose-response curves severely rightward. Time-control experiments (C) demonstrate that the responses to adenosine are stable over time. Concentrations indicate the concentration in the 25-μl bolus injectate. The width of the horizontal bar on the experimental dose-response curve indicates the difference in rightward shift in the 2 hearts. *P < 0.01 vs. the adenosine time-control experiment.

Fig. 3.

Responses to intracoronary bolus AMP injections. A: the P1 receptor antagonist 8-SPT greatly reduces AMP vasodilation. B: adenosine deaminase in the perfusate has no significant effect on AMP vasodilation, suggesting that AMP acts independently from adenosine. C: when adenosine responses are eliminated by adenosine deaminase, 8-SPT remains an effective AMP antagonist, confirming that AMP dilates independently from adenosine and acts via P1 receptors. D: the effects of 8-SPT are not due to time. Concentrations indicate the concentration in the 25-μl bolus injectate. The width of the horizontal bar on the experimental dose-response curve indicates the difference in rightward shift in the 2 hearts. *P < 0.01 vs. the AMP time-control experiment.

Fig. 4.

Responses to intracoronary bolus ADP injections. A: P1 receptor blockade by 8-SPT shifts the ADP dose-response curve moderately to the right, indicating some contribution by AMP and/or adenosine. B: after P1 receptor blockade with 8-SPT, 50 μM N⁶-methyl-2′-deoxyadenosine 3′,5′-bisphosphate (MRS 2179) severely inhibits vasodilation to 2-methylthio-ADP (2-MeS-ADP), confirming that MRS 2179 is an effective P2Y₁ receptor antagonist. C: after P1 receptor blockade with 8-SPT, MRS 2179 shifts the ADP dose-response curve further rightward, indicating a contribution by P2Y₁ receptors. The P2Y₁ component is especially prominent at low ADP concentrations, where P1 receptor blockade has little effect (A). D: ADP responses are stable over time. Concentrations indicate the concentration in the 25-μl bolus injectate. The width of the horizontal bar on the experimental dose-response curve indicates the difference in rightward shift in the 2 hearts. *P < 0.01 vs. the ADP time-control experiment.

Fig. 5.

Responses to intracoronary bolus ATP injections. A: P1 receptor blockade by 8-SPT inhibits ATP-induced vasodilation, indicating a contribution by AMP and/or adenosine. B: adenosine deaminase does not inhibit responses to ATP, indicating lack of contribution by adenosine. C: when adenosine responses are eliminated by adenosine deaminase and P2Y₁ receptor-mediated vasodilation is blocked by MRS 2179, 8-SPT again inhibits responses to ATP. This indicates that a significant amount of ATP-induced vasodilation is due to AMP acting at P1 receptors. D: ATP responses are stable over time. Concentrations indicate the concentration in the 25-μl bolus injectate. The width of the horizontal bar on the experimental dose-response curve indicates the difference in rightward shift in the 2 hearts. *P < 0.01 vs. the ATP time-control experiment.

Fig. 6.

The role of P2Y₁ receptors in ATP-induced coronary vasodilation. A: in the presence of P1 blockade with 8-SPT, 2-methylthio-ATP (2-MeS-ATP) causes vasodilation that is antagonized by the P2Y₁ antagonist MRS 2179. B: after P1 receptor blockade with 8-SPT, MRS 2179 provides further inhibition of ATP-induced vasodilation, indicating a P2Y₁ receptor contribution. C: P1 receptor blockade by 8-SPT is not strengthened by addition of MRS 2179, indicating that the results in B are due to P2Y₁ receptor blockade. D: combined P1 and P2Y₁ receptor blockade inhibits ATP-induced coronary vasodilation. Concentrations indicate the concentration in the 25-μl bolus injectate. The width of the horizontal bar on the experimental dose-response curve indicates the difference in rightward shift in the 2 hearts. *P < 0.01 vs. the ATP time-control experiment; †P < 0.01 vs. experiment in C.