Adenosine A3 receptor mediated coronary vasodilation in the rat heart: Changes that occur with maturation

Adenosine A2B and A3 receptors (ADOR) have been reported to induce coronary vasodilation in the rat. This study investigated the effect of age on ADORA3 mediated coronary responses using hearts from rats aged 6-8 weeks (immature), 16-18 weeks (young) and 52-54 weeks (mature) perfused in Langendorff mode. APNEA (ADORA3 > ADORA1 agonist) was observed to activate at least two receptor subtypes to mediate a biphasic vasodilator response in hearts from immature rats. The potency of APNEA at the high affinity site was enhanced by alloxazine (ADORA2B antagonist) and reduced when combined with MRS1191 (ADORA3 antagonist). This indicates that the high affinity phase is the ADORA3, and ADORA2B signalling is likely to play a negative regulatory role towards the ADORA3 mediated response. The activity at this site was also reduced with maturation. The low affinity site was inhibited by alloxazine but not MRS1191, indicating that this response is mediated by the ADORA2B or another receptor subtype. The response at this site did not alter with age. Cl-IB-MECA (ADORA3 agonist) produced monophasic responses that were inhibited by alloxazine but remained unaffected by MRS1191 in all age groups. In addition the potency of Cl-IB-MECA does not change in hearts from PTX-treated rats. However, the maximal responses increased, indicating Gi protein independent and dependent signalling. Q-PCR analysis of rat hearts indicated the presence of an ADORA3 splice variant (ADORA3i), which increased in mRNA expression with age. Cl-IB-MECA responses may be mediated by this ADORA3i. In conclusion, APNEA mediates coronary vasodilation in the rat heart via at least two receptor sites, the ADORA3 and ADORA2B. ADORA3 responses are reduced while ADORA2B remain unchanged with maturation. In addition, the splice variant ADORA3i may contribute to coronary responses in the rat heart.