CALCIUM MOBILIZATION AND ISOMETRIC TENSION IN BOVINE TRACHEAL SMOOTH-MUSCLE - EFFECTS OF SALBUTAMOL AND HISTAMINE

We determined if decreases in relative free intracellular calcium conc entration ([Ca2+](i)) caused by salbutamol, a selective beta(2)-adreno receptor agonist, were paralleled by calcium egression from the cytoso l in bovine trachealis muscle strips. [Ca2+](i), or tissue-surface ext racellular calcium changes (Ts[Ca2+](ext)), were monitored using Fluo- 3 acetoxymethylester or Fluo-3 pentaammonium salt simultaneously with isometric tension. Salbutamol (1 mu M) decreased histamine-induced iso metric tension from an average peak tension of 128.5 +/- 18.4 to -4.9 +/- 0.3 mN/mm(2), and reduced the associated sustained increases in [C a2+](i) from 100% at peak to 20.4 +/- 7.6%. Both histamine-induced ele vation in [Ca2+](i) and isometric tension were reversed completely by forskolin (1 CIM) In muscle strip at active resting tension, salbutamo l caused a decrease (49.6 +/- 12.1%) in [Ca2+](i). Following precontra ction with histamine, salbutamol caused an immediate and sustained inc rease in Ts[Ca2+](ext) which was not seen in a Na+-free solution. Fina lly, propranolol (10 mu M) blocked both increases in Ts[Ca2+](ext) and muscle relaxation caused by salbutamol. These findings indicate that in bovine trachealis muscle, the effect of salbutamol to decrease [Ca2 +](i) and isometric tension is via a beta(2)-adrenoceptor, and the cha nges in [Ca2+](i) are by an increase in calcium egression via the Na+/ Ca2+ exchanger, and reuptake by myoplasmic stores.