Effects of sodium and calcium concentrations on the potentiation by indomethacin of the responses of rabbit mesenteric and coeliac arteries to vasoconstrictor agonists

Differential regulation of norepinephrine- and prostaglandin F2alpha-induced contraction by extracellular Na+ in rat aorta

The purpose of this study was to test whether extracellular Na+ differentially regulates agonist-induced contraction in vascular smooth muscle. Exposure of rat aorta to 20 nM extracellular Na+ by substitution of 123 mM Na+ with N-methyl-D-glucamine or choline, inhibited norepinephrine-induced contraction to a greater magnitude than contraction to prostaglandin F2alpha. In the absence of extracellular Ca2+ and in 20 mM Na+ solution containing 123 mM N-methyl-D-glucamine, the norepinephrine and prostaglandin F2alpha contraction remained unaltered. In contrast, in the absence of extracellular Ca2+ and in 20 mM Na+ solution containing 123 mM choline, the norepinephrine and prostaglandin F2alpha contraction were decreased and increased, respectively. Contraction to the phorbol ester, phorbol dibutyrate, was inhibited in 20 mM extracellular Na+ solution containing N-methyl-D-glucamine. Removal of extracellular Ca2+ inhibited the phorbol dibutyrate contraction, and 20 mM extracellular Na+ solution containing N-methyl-D-glucamine did not inhibit the phorbol dibutyrate contraction elicited in the absence of extracellular Ca2+. Complete replacement of extracellular Na+ with choline, and concomitant treatment with nifedipine to reduce the elevated basal tone after Na+ replacement, also resulted in greater inhibition of norepinephrine- as compared with prostaglandin F2alpha-induced contraction. Ethylisopropylamiloride, a Na+/H+ exchange inhibitor, did not alter norepinephrine contraction, as determined in the presence of nifedipine to reduce the elevated basal tone due to ethylisopropylamiloride. Acidification, which may result from decreased Na+/H+ exchange, inhibited the prostaglandin F2alpha-induced contraction to a greater magnitude than contraction to norepinephrine. These results demonstrate that extracellular Na+ selectively regulates agonist-induced contraction. The study further suggests that the selectivity may be related to an extracellular Na+-dependent process that is activated by protein kinase C, such as Na+/Ca2+ exchange, and is unrelated to the release of intracellular Ca2+ and Na+/H+ exchange.

Regulation of vascular smooth muscle contraction by extracellular Na+

1. The effects of extracellular Na+ removal on agonist-induced contraction of vascular smooth muscle in vitro are reviewed. 2. The effects of extracellular Na+ removal on contraction vary depending upon the agonist and vessel. 3. Factors that may influence the effects of extracellular Na+ removal on agonist-induced contraction include the compound substituted for Na+, time of tissue exposure to lowered extracellular Na+, concentration of extracellular Na+, agonist concentration, presence of the vascular endothelium and presence of basal tone. 4. The potential influence of these factors needs to be determined and minimized, in studies that investigate the role of extracellular Na+ in agonist-induced contraction.

Responsiveness of rat vas deferens and stomach smooth muscles after treatment with indomethacin

The contractile responses of rat vas deferens and longitudinal stomach fundus strips to stimulants and relaxants were studied after treatment with indomethacin (Ind) injected i.p. at doses of 15 mg/kg for 24 hr and 25 mg/kg for 48 hr. The treatment with Ind reduced the amount of both spontaneously and electrically-evoked release of a prostaglandin (PG)-like substance from the isolated n. hypogastricus-vas deferens preparations. A considerable decrease of the responsiveness to exogeneous noradrenaline occurred in vas deferens isolated from Ind-pretreated rats. The EC50 values of noradrenaline increased as compared to the control value. The contractile responses of the isolated longitudinal stomach fundus strips to acetylcholine, serotonin, PGE2, PGF2 and arachidonic acid were reduced while the relaxant action of noradrenaline and papaverine was potentiated after Ind treatment. It is assumed that endogeneous PGs participate in the regulation of smooth muscle activity through potentiation of the postjunctional excitatory action of noradrenaline in vas deferens and through interaction with the transmembrane ion transport in stomach fundus smooth muscles.

Endothelium-dependent inhibition of the use of extracellular calcium for the arterial response to vasoconstrictor agents

The responses of rabbit mesenteric or coeliac artery rings to angiotensin II or adrenaline (but not to K+) were enhanced by endothelium destruction (by rubbing). Potentiation by indomethacin of the response to the agonists was observed in rubbed rings but not in intact ones. Both angiotensin II and adrenaline (in the presence of propranolol and prazosin) induced endothelium-dependent relaxation of the arteries. Rubbed rings, but not intact ones, contracted when Ca2+ was added to a previously Ca2+-free medium containing angiotensin II or adrenaline. The vasoconstrictor response appears to be modulated by the regulation of receptor-operated Ca2+ channels through EDRF released by the endothelium and by some cyclo-oxygenase product at the level of the smooth muscle cell membrane.