Protein free diet feeding: Effects on sympathetic activity and salivary evoked secretion in the submandibular gland of the rat

After 21 days, rats fed casein 0% presented: (a) a significant shift to the left of the dose-response curves (DRC) to the autonomic agonists-norepinephrine (NE), methoxamine, isoproterenol (ISO) and methacholine; (b) increased food consumption (p < 0.001); (c) decreased body (p < 0.001) and SMG (p < 0.001) weights maintaining SMG/body (w/w) relation; (d) enhanced submandibular α1-adrenoceptor number without changes in the apparent dissociation constant (Kd); (e) increased submandibular NE content (p < 0.05) and phosphoinositoside hydrolysis (p < 0.001); (f) decreased submandibular tyrosine hydroxylase activity (TH) (p < 0.01). Casein 5% feeding increased food consumption (p < 0.01) and reduced body weight (p < 0.05). This protein restriction increased metacholine-evoked salivation, but it altered neither submandibular sympathetic activity nor sympathetic-induced salivary secretion as compared to the Control group (C) fed a similar diet containing 25.5% protein. Present results suggest that in the adult rat, a protein free diet during 21 days lowers SMG sympathetic and cholinergic activity leading to supersensitivity as revealed by up-regulation of α1-adrenergic receptor number and increased autonomic-evoked salivation.