EFFECT OF ISOPRENALINE AND PHENYLEPHRINE ON THE ADENOSINE 3′,5′‐MONOPHOSPHATE CONTENT AND MECHANICAL ACTIVITY OF COLD‐STORED AND FRESH TAENIA CAECUM FROM THE GUINEA‐PIG

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Abstract
1 Cold storage treatment of the guinea-pig taenia caecum had a greater inhibitory effect on the isoprenaline-induced relaxation than that induced by phenylephrine. Prolonged cold storage (12-14 days) almost abolished the effect of isoprenaline but only reduced the phenylephrine effect. The ED50 of cyclic adenosine 3′,5′-monophosphate (cyclic AMP) that elicited muscle relaxation was not altered by the prolonged cold storage. 2 After cold storage treatment, tissue cyclic AMP content was decreased; however, isoprenaline still caused a dose-dependent increase in the cyclic AMP level. The threshold dose of isoprenaline for cyclic AMP accumulation was the same in fresh and cold-stored preparations. 3 In the fresh preparation, the onset of the isoprenaline (10−6 M)-induced relaxation preceded the increase in tissue cyclic AMP. 4 Isoprenaline, phenylephrine, adrenaline and noradrenaline at doses (ED50) sufficient to induce muscle relaxation did not always increase the cyclic AMP level. 5 Similarly, the responses to papaverine and nitroglycerine were not accompanied by an increase in cyclic AMP. 6 The adenylate cyclase and phosphodiesterase (low and high Km) activities of taenia caecum were not attenuated by the prolonged cold storage. 7 Propranolol inhibited both the isoprenaline-induced relaxation and cyclic AMP accumulation; however, the pA2 values were significantly different for the two events. 8 Based on these results, both the relaxation and cyclic AMP accumulation caused by isoprenaline are mediated by activation of β-adrenoceptors but are independent phenomena.

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