Acidosis reduces the function and expression of α1D-adrenoceptor in superior mesenteric artery of Capra hircus

OBJECTIVE The objective of this study was to characterize the α1-adrenoceptor (α1-AR) subtypes and evaluate the effect of acidosis on α1-AR function and expression in goat superior mesenteric artery (GSMA). MATERIALS AND METHODS GSMA rings were mounted in a thermostatically controlled (37.0°C ± 0.5°C) organ bath containing 20 ml of modified Krebs-Henseleit solution, maintained at pHo of 7.4, 6.8, 6.0, 5.5, 5.0, and 4.5. Noradrenaline (NA)- and phenylephrine (PE)-induced contractile response was elicited in the absence or presence of endothelium and prazosin at pHo of 7.4, 6.0, and 5.0. The responses were recorded isometrically by an automatic organ bath connected to PowerLab and analyzed using Labchart 7.1.3 software. Expression of α1D-AR was compared at physiological and acidic pHo using reverse transcription-polymerase chain reaction (RT-PCR). RESULTS NA- and PE-induced contractile responses were attenuated proportionately with a decrease in extracellular pH (pHo), i.e. 7.4 → 6.8 → 6.0 → 5.5 → 5.0 → 4.5. Endothelium denudation increased the contractile response at both normal and acidic pHo. Prazosin (1 nM, 10 nM, and 0.1 μM) inhibited the NA- and PE-induced contractile response at pHo 7.4 and the blocking effect of prazosin was potentiated at pHo of 6.0 and 5.0. RT-PCR analysis for α1D-AR in GSMA showed that the mRNA expression of α1D-AR was decreased under acidic pHo as compared to physiological pHo. CONCLUSION (i) Adrenergic receptor mediates vasoconstriction in GSMA under normal physiological pHo, and α1D is the possible subtype involved in this event (ii) acidosis attenuates the vasocontractile response due to reduced function and expression of α1D-AR and also increased the release of endothelial-relaxing factors.