Functional characterization of post-junctional adrenergic receptor subtypes in bovine intra-mammary arteries

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 pH_o 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 pH_o 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 α_1D-AR was compared at physiological and acidic pH_o using reverse transcription-polymerase chain reaction (RT-PCR).

Results:

NA- and PE-induced contractile responses were attenuated proportionately with a decrease in extracellular pH (pH_o), 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 pH_o. Prazosin (1 nM, 10 nM, and 0.1 μM) inhibited the NA- and PE-induced contractile response at pH_o 7.4 and the blocking effect of prazosin was potentiated at pH_o of 6.0 and 5.0. RT-PCR analysis for α_1D-AR in GSMA showed that the mRNA expression of α_1D-AR was decreased under acidic pH_o as compared to physiological pH_o.

Conclusion:

(i) Adrenergic receptor mediates vasoconstriction in GSMA under normal physiological pH_o, and α_1D is the possible subtype involved in this event (ii) acidosis attenuates the vasocontractile response due to reduced function and expression of α_1D-AR and also increased the release of endothelial-relaxing factors.

Adrenergic receptors in the bovine mammary artery

The response of isolated preparations of bovine mammary artery was investigated, with the aim of characterising further the adrenergic receptor subtypes present. Noradrenaline (NA) and the alpha(1) agonist phenylephrine gave sigmoidal dose-response curves with pEC(50) values of 5.97+/-0.07 (N=34) and 6.21+/-0.32 (N=8), respectively. Stimulation of alpha(2) receptors with UK 14,304 produced a weak response with pEC(50) of 6.78+/-0.38 (N=7), and maximal contraction of 17.8+/-9.9% relative to NA. A61603, an alpha(1A) agonist, gave a curve parallel to NA, but shifted to the left (pEC(50) of 6.98+/-0.19 (N=5)); this drug had an increased potency of 10-fold relative to NA, and 4-fold relative to phenylephrine. Schild analysis of curves obtained with the alpha(1) antagonist prazosin gave a pA(2) of 8.70+/-0.47 (N=6-9), whereas the alpha(2) antagonist yohimbine resulted in a pA(2) of 7.65+/-0.16 (N=4). The alpha(1A) receptor antagonists WB4101 and 5-methylurapidil gave pA(2) values of 9.39+/-0.69 (N=4) and 7.72+/-0.02 (N=2-3), respectively. The irreversible alpha(1B) inhibitor CEC reduced the pEC(50) from 5.39+/-0.12 to 4.31+/-0.18 (N=7) only at the highest dose used, and high doses of the alpha(1D) antagonist BMY 7378 produced a shift to the right at giving a pA(2) of 7.37+/-0.08 (N=3). These results suggest major involvement of the alpha(1B) adrenergic receptor subtype in contraction of the bovine mammary artery, which is similar to the human internal mammary artery.