A Blunted Sympathetic Function and an Enhanced Nitrergic Activity Contribute to Reduce Mesenteric Resistance in Hyperthyroidism

We aimed to determine whether an experimental model of hyperthyroidism could alter the function of sympathetic and nitrergic components of mesenteric innervation. For this purpose, male Wistar rats were divided into (1) control rats (CT) and (2) rats infused with L-Thyroxine (HT). Body weight gain and adipose tissue accumulation were lower in HT rats, while systolic blood pressure and citrate synthase activity in the soleus muscle were increased by HT. In segments from the superior mesenteric artery, the application of an electrical field stimulation (EFS) induced a vasoconstrictor response, which was lower in arteries from HT animals. The alpha-adrenoceptor antagonist phentolamine diminished EFS-induced vasoconstriction to a lower extent in HT arteries, while the purinergic receptor antagonist suramin reduced contractile response to EFS only in segments from CT. In line with this, noradrenaline release, tyrosine hydroxylase expression and activation and dopamine β hydroxylase expression were diminished in HT. The unspecific nitric oxide synthase (NOS) inhibitor L-NAME increased EFS-induced vasoconstriction more markedly in segments from HT rats. NO release was enhanced in HT, probably due to an enhancement in neuronal NOS activity, in which a hyperactivation of both PKC and PI3K-AKT signaling pathways might play a relevant role. In conclusion, perivascular mesenteric innervation might contribute to reduce the vascular resistance observed in hyperthyroidism.