Daily changes in β-adrenergic sensitivity of rat submandibular gland. Correlation with β-adrenoceptor rhythm.

Neurons from the superior cervical ganglia (SCG) innervate the submandibular gland and release noradrenaline during the dark phase of the daily photoperiod. Since in the pineal, another structure innervated by sympathetic neurons, nocturnal activation of the SCG is associated with β-adrenergic sub- and super-sensitivity rhythms, the possible existence of similar phenomena in the rat submandibular gland was assessed. Wistar female rats, kept on a 14:10 light/dark cycle (light from 06:00 to 20:00 h), were sacrificed at 09:00, 14:00, 20:00, 24:00 and 04:00 h. β-Adrenoceptors were studied by 3H-dihydroalprenolol binding to membrane preparations. The equilibrium dissociation constant (Kd) did not change as a function of time while significant daily variations in maximal binding values (Bmax) were observed with a peak at 20:00 h. Changes in Bmax correlated with a high response of adenylate cyclase to isoproterenol. In addition, when the response in salivary flow to isoproterenol was measured, a shift to the left (about 1 logarithmic unit) in dose–response curves was observed at 19:00–20:00 h as compared to 08:00–09:00 h. These daily variations in isoproterenol responsiveness seem not to depend on the pattern of eating since a 24-h starvation or a nocturnal starvation for 16–18 days did not abolish the morning–evening differences in the salivary flow response to isoproterenol. Rather, the results suggest that the daily variations in isoproterenol response correlate with β-adrenergic super- and sub-sensitivity phenomena associated with the circadian release of noradrenaline from SCG neurons.