PHENTOLAMINE AND HYPOXIA - MODULATION OF CONTRACTILITY AND ALPHA(1)-ADRENOCEPTORS IN ISOLATED RAT ATRIA

The hypoxia-induced effects on the binding sites and affinity constant of adrenoceptors, in the presence and absence of phentolamine, were determined for atrial membranes of hearts from normal and genetically hyperlipidaemic Yoshida (YOS) rats. Atrial function was also measured during normoxia and hypoxia, in the presence and absence of phentolamine. Hypoxia increased alpha 1-adrenoceptor density in atrial membranes of normal rats (Bmax 10.6 to 26.7 fmoles/mg protein). Phentolamine prevented the increase in the Bmax of alpha 1-adrenoceptors and increased the equilibrium dissociation constant of these receptors (KD 0.17 to 0.53 nmol/l). Beta-adrenoceptors did not change during hypoxia, but the Bmax was slightly increased (26%) in the presence of phentolamine. Thus, the alpha 1/beta ratio increased from 0.40 in normoxia to 1.06 in hypoxia. In normoxic atria from YOS rats, the alpha 1/beta ratio was already elevated (0.86) in comparison to control rats (mainly due to a higher density of alpha 1-adrenoceptors in atrial membranes from YOS rats). This ratio was not modified by hypoxia (0.84), but decreased when phentolamine was present (0.30). Hypoxia reduced the force of contraction and increased diastolic tension of atria of normal rats, while the sinus rate was not significantly modified. Phentolamine abolished the increase in diastolic tension and reduced the negative effect of hypoxia on contractile force. In YOS rat atria, functional parameters were modified by hypoxia in a qualitatively similar way to that of normal rat atria