Stimulation of sodium pump by vasoactive intestinal peptide in guinea-pig isolated trachea: Potential contribution to mechanisms underlying relaxation of smooth muscle

Abstract

1. Relaxation of airway smooth muscle induced by vasoactive intestinal peptide (VIP) is mediated by adenosine 3':5' cyclic monophosphate (cyclic AMP). An interaction between the synthesis of cyclic AMP and enzymic activity of the plasmalemmal sodium pump (Na+-K+-ATPase) exists in certain isolated cell systems. This study sought to determine the contribution of Na+-K+-ATPase activity to relaxation of airway smooth muscle evoked by VIP. 2. All experiments were performed on isolated strips of guinea-pig trachea from which the epithelium had been removed. VIP was a more potent relaxant in tissues that were contracted with carbachol than those contracted with an equi-effective depolarizing concentration of K+. 3. Ouabain (0.1 μM-10 μM) induced contraction of tracheal strips. Contraction to ouabain (5 μM) was abolished following incubation of tissues with K+-free, or Ca2+-free (+EGTA, 0.1 mM) physiological solutions. The contractile response to ouabain (5 μM) was not influenced significantly by exposure of the tissues to atropine (1 μM), phentolamine (5 μM) and diphenhydramine (1 μM) for 60 min. 4. Tissues were incubated with ouabain (5 μM; 60 min) or K+-free physiological solution (60 min) to inhibit Na+-K+-ATPase activity. These procedures reduced relaxation induced by VIP, peptide histidine isoleucine, forskolin, isoprenaline and sodium nitroprusside. 5. Relaxation to VIP was impaired significantly following exposure of tissues to a low Na+ solution (30 min) or amiloride (500 μM; 30 min). 6. Ouabain-sensitive uptake of 86Rb was measured in tracheal strips (devoid of epithelium and cartilage) as an index of Na+-K+-ATPase activity. VIP (1 μM; 2 min) caused a 4.7 fold stimulation of ouabain-sensitive uptake of 86Rb. This effect was impaired significantly by low Na+ solution. 7. The results suggest that (i) relaxation of tracheal smooth muscle to VIP is sensitive to procedures that inhibit activity of Na+-K+-ATPase and invoke a role for altered sodium pump function in the mechanisms that underlie cyclic AMP-dependent relaxation; and (ii) VIP stimulates ouabain-sensitive uptake of 86Rb in airway smooth muscle in a Na+-dependent manner.