Effect of extracellular pH on contractile responses of the guinea-pig vas deferens

Acidosis attenuates P2X purinergic vasoconstriction in skeletal muscle arteries

Vasoconstriction via alpha(2)-receptors is known to be sensitive to acidic pH, but little is known about the pH sensitivity of P2X receptors. ATP is a cotransmitter released with norepinephrine from the sympathetic nerves and causes vasoconstriction via P2X purinergic receptors on vascular smooth muscle. We hypothesized that reductions in pH would attenuate P2X-mediated vasoconstriction in iliofemoral artery rings. Twenty-five rats were killed, and the iliac and femoral arteries were dissected out and placed in modified Krebs-Henseleit buffer. The arteries were cut into 2-mm sections and mounted in an organ tissue bath. Tension (g) was measured during a potassium chloride and norepinephrine challenge (maximal tension). The arteries were then exposed to alpha,beta-methylene ATP (10(-7)-10(-3) M; n = 13) or phenylephrine (10(-7)-10(-4) M; n = 6) with a tissue bath pH of 7.8, 7.4, and 7.0. Dose-response curves were fit with nonlinear regression analysis to calculate the EC(50) and slope. The peak tension with alpha,beta-methylene ATP was lower during pH 7.0 (1.37 +/- 0.09 g) compared with pH 7.8 (1.90 +/- 0.12 g). EC(50) was highest with pH 7.4 (-5.38 +/- 0.18 log M alpha,beta-methylene ATP) and lowest with pH 7.0 (-4.9 +/- 0.10 log M alpha,beta-methylene ATP). The slopes of the dose-response curves were not different. Pyridoxal phosphate-6-azo(benzene-2,4-disulfonic acid) abolished contraction caused by the addition of alpha,beta-methylene ATP (n = 6). There was no effect of pH on phenylephrine dose-response curves. These data indicate that the vasoconstrictor response to alpha,beta-methylene ATP is sensitive to pH and that lower pH attenuates the response of P2X purinergic receptors.

Temperature dependency of P2 receptor-mediated responses

The P2 receptor-mediated responses of isolated guinea pig urinary bladder and vas deferens (P2X receptors) and taenia caeci (P2Y receptors) were registered at the three temperature conditions of 30, 37 and 42 degrees C. The contractile responses of both urinary bladder and vas deferens to a P2X receptor agonist alpha,beta-methylene ATP (alpha,beta-meATP; 0.01-30 microM) and to electrical field stimulation (1-64 Hz, 0.1 ms, supramaximal voltage) in the presence of atropine (0.1 microM) and phentolamine (1 microM) were markedly more prominent at a temperature of 30 degrees C than at 37 or 42 degrees C. Similarly, relaxation of carbachol-precontracted taenia caeci caused by electrical field stimulation (0.5-8 Hz, 0.1 ms, supramaximal voltage) temperature-dependently increased with decrease of temperature, while relaxation of this tissue by exogenous ATP (1-100 microM) was not affected by the temperature. A P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 1-30 microM) at all three temperature conditions concentration-dependently antagonised contractile responses to alpha,beta-methylene ATP and electrical field stimulation in both urinary bladder and vas deferens. PPADS, even at the highest concentration tested (30 microM), had no effect on the relaxant responses of the taenia caeci either to electrical field stimulation or ATP and its action was not affected by the change of temperature. It is concluded from this study that the effectiveness of P2 receptor-mediated responses in guinea pig urinary bladder, vas deferens and taenia caeci increases by decrease of temperature.

Heteromultimeric P2X(1/2) receptors show a novel sensitivity to extracellular pH

Rat P2X(1) and P2X(2) subunits were coexpressed in defolliculated Xenopus oocytes and the resultant P2X receptors studied under voltage-clamp conditions. Extracellular ATP elicited biphasic inward currents, involving an initial rapidly inactivating (P2X(1)-like) component and a later slowly inactivating (P2X(2)-like) component. The maximum amplitude of P2X(1)-like ATP responses was increased in some cells by lowering extracellular pH (from 7.5 to 6.5), whereas P2X(2)-like responses and those of homomeric rP2X(1) and rP2X(2) receptors were not changed by this treatment. Concentration-response (C/R) curves for ATP for pH-enhanced P2X(1)-like responses were biphasic, and clearly distinct from monophasic ATP C/R curves for homomeric rP2X(1) and rP2X(2) receptors. Under acidic (pH 5.5 and 6.5) and alkaline (pH 8.5) conditions, ATP C/R curves for P2X(1)-like responses showed increases in agonist potency and efficacy, compared with data at pH 7.5, but the same was not true of homomeric rP2X(1) and rP2X(2) receptors. ATP C/R curves for P2X(2)-like responses overlay C/R curves for homomeric rP2X(2) receptors, and determinations of agonist potency and efficacy were identical for P2X(2)-like and P2X(2) responses at all pH levels tested. Our results show that P2X(1)-like responses possessed the kinetics of homomeric P2X(1) receptors but an acid sensitivity different from homomeric P2X(1) and P2X(2) receptors. In contrast, the P2X(2)-like responses exactly matched the profile expected of homomeric P2X(2) receptors. Thus, coexpression of P2X(1) and P2X(2) subunits yielded a mixed population of homomeric and heteromeric P2X receptors, with a subpopulation of novel pH-sensitive P2X receptors showing identifiably unique properties that indicated the formation of heteromeric P2X(1/2) ion channels.