The role of α- and β-adrenoceptor subtypes in mediating the effects of catecholamines on fasting glucose and insulin concentrations in the rat

The role of α- and β-adrenoceptor subtypes in the regulation of plasma glucose and immunoreactive insulin (IRI) levels has been investigated in normal conscious fasted rats by employing selective agonists and antagonists. Adrenaline (0.2 mg kg^-1)-induced hyperglycaemia was abolished by the selective α₂-adrenoceptor antagonist, idazoxan (1.0 mg kg^-1), unaltered by non-selective β-adrenoceptor blockade (propranolol, 1.0 mg kg^-1) and potentiated by the selective α₁-adrenoceptor antagonist prazosin (0.3 mg kg^-1). Adrenaline increased plasma IRI levels in the presence of idazoxan but not in the presence of either prazosin or propranolol. The selective α₂-adrenoceptor agonists UK 14304 (0.1 and 0.3 mg kg^-1) and BHT-920 (0.2 and 0.5 mg kg^-1) elicited dose-dependent hyperglycaemic responses, but did not alter plasma IRI levels. UK 14304 (0.1 mg kg^-1)-evoked hyperglycaemia was blocked by idazoxan but not by prazosin. The selective α₁-adrenoceptor agonists methoxamine (0.3 mg kg^-1) and phenylephrine (0.3 mg kg^-1) failed to modify either plasma glucose or IRI levels. Isoprenaline (0.2 mg kg^-1) elicited hyperglycaemic and insulinotropic responses which were attenuated by propranolol (1.0 mg kg^-1) and the selective β₂-adrenoceptor antagonist ICI 118551 (1.0 mg kg^-1), but not by the β₁-selective antagonists atenolol (1.0 mg kg^-1) and betaxolol (1.0 mg kg^-1). None of the antagonists per se affected basal plasma glucose or IRI concentrations, except prazosin (1.0 mg kg^-1). The results indicate that adrenoceptors do not appear to be involved in regulating basal plasma glucose and IRI concentrations in the fasted rat. However, the effects of catecholamines on these parameters are mediated by α₂- and β₂-adrenoceptors, whereas α₁- or β₁-adrenoceptors do not appear to be involved.