EFFECT OF DIFFERENT TREATMENTS IN CALCIUM-FREE MEDIUM ON BASAL TONE AND CONTRACTILE RESPONSES OF GUINEA-PIG TRACHEAS

Acetylcholine (ACh; 0.1 mmol/l) and KCl(80 mmol/l) induce a biphasic c ontractile response in isolated guinea pig tracheae maintained at 37 d egrees C either in the presence or absence of extracellular Ca2+. Expo sure of the tissue to Ca2+-free solution evokes a significant decrease in basal tone and the sources of Ca2+ appear to be decreased by prolo nged agonist stimulation, and even more by successive agonist stimulat ion. After an incubation period of 20 min in Ca2+-containing solution, the response is restored. Mg2+-depletion in Ca2+-free medium increase d the contractile response to ACh, but not to KCl, and delayed the ton ic component of the next contraction elicited in Ca2+-containing solut ion.