Abstract
1. 1. Cooling of guinea-pig isolated atria from 38 to 25°C induced supersensitivity of the tension and rate responses to isoprenaline as determined by EC 50 values. 2. 2. When plotted in absolute units, only the tension responses exhibited supersensitivity. 3. 3. Using tropolone as a catechol- O-methyl transferase (COMT) inhibitor, or orciprenaline as the agonist, COMT inhibition by cooling was found to be entirely responsible for rate supersensitivity but only partially for tension. 4. 4. The residual tension supersensitivity to orciprenaline was not due to extraneuronal uptake or phosphodiesterase inhibition by cooling. 5. 5. CaCl 2 and ouabain tension responses were not potentiated, suggesting that it is not a post-β-adrenoceptor phenomenon.
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