Calcium mobilization and isometric tension in bovine tracheal smooth muscle: effects of salbutamol and histamine

We determined if decreases in relative free intracellular calcium concentration ([Ca 2+ ] i ) caused by salbutamol, a selective β 2 -adrenoreceptor agonist, were paralleled by calcium egression from the cytosol in bovine trachealis muscle strips. [Ca 2+ ] i, or tissue-surface extracellular calcium changes (Ts[Ca 2+ ] ext were monitored using Fluo-3 acetoxymethylester or Fluo-3 pentaammonium salt simultaneously with isometric tension. Salbutamol (1 μM) decreased histamine-induced isometric 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 [Ca 2+ ] i from 100% at peak to 20.4 ± 7.6%. Both histamine-induced elevation in [Ca 2+ ] i and isometric tension were reversed completely by forskolin (1 μM). In muscle strip at active resting tension, salbutamol caused a decrease (49.6 ± 12.1 %) in [Ca 2+ ] i. Following precontraction with histamine, salbutamol caused an immediate and sustained increase in Ts[Ca 2+ ] ext which was not seen in a Na + -free solution. Finally, propranolol (10 μM) blocked both increases in Ts[Ca 2+ ] ext and muscle relaxation caused by salbutamol. These findings indicate that in bovine trachealis muscle, the effect of salbutamol to decrease [Ca 2+ ] i and isometric tension is via a β 2 -adrenoceptor, and the changes in [Ca 2+ ] i, are by an increase in calcium egression via the Na+Ca2+ exchanger, and reuptake by myoplasmic stores.