Depressive cardio-respiratory effects of somatostatin in anaesthetized rats.

Cardio-respiratory effects of intravenous injection of somatostatin were investigated in anaesthetized, spontaneously breathing rats that were: (i) neurally intact and subsequently bilaterally vagotomized in the neck, or (ii) midcervically vagotomized with later vagotomized at the supranodosal level, or (iii) midcervically vagotomized and subjected to section of the carotid sinus nerves (CSNs). Intravenous injection of 100mug/kg of somatostatin before and after midcervical vagotomy-induced immediate slowing down of the respiratory rate and decreased tidal volume, mean arterial blood pressure (MAP) and heart rate. Supranodose vagotomy did not abolish somatostatin-induced respiratory depression. CSNs section eliminated all respiratory effects of somatostatin challenge. In all the neural states, somatostatin caused significant falls in mean arterial blood pressure and heart rate. The results of this study suggest that hypoventilation induced by intravenous somatostatin administration occurs outside vagal afferentation to the medulla and is mediated via carotid body afferents. Sino-aortic chemoreceptors and baroreceptors do not contribute to bradycardia and a fall in blood pressure. These cardiovascular effects are presumably mediated due to excitation of somatostatin receptors within the heart and/or in the brain.