No abstract

Colorectal distensions of 60 and 80 mmHg significantly reduced heart rate (HR) and cardiac sympathetic nerve activity in anesthetized rats. This bradycardiac response was not influenced by the intravenous administration of atropine, but was abolished by propranolol, suggesting that it was elicited by sympathetic but not vagal efferent nerves.Key words: colorectal distension, heart rate, cardiac sympathetic nerve activity.It has been reported that cardiovascular function is reflexively regulated by visceral noxious stimulation, such as distension of stomach [1], colorectum [2], biliary system [3], and uterus [4], in anesthetized animals. Our recent study demonstrated that a noxious range of colorectal distension (CRD) produced a reflexive decrease in blood pressure in anesthetized central nervous system (CNS)-intact rats via a reflexive inhibition of sympathetic vasoconstrictor nerve (renal sympathetic nerve) activity [5]. The decrease response of blood pressure induced by CRD in CNS-intact rats was reversed to increase response by spinalization at the 2nd cervical level (C2). Thus it was suggested that the reflex center of the CRD-induced inhibitory reflex modulation of blood pressure was at supraspinal levels in CNS-intact rats. There was also CRD-induced excitatory reflex modulation of blood pressure at the propriospinal levels, though the excitatory reflex was inhibited by the supraspinal structure in CNS-intact preparation [5].During these experiments on the blood pressure, we had observed that CRD produced bradycardia in association with a decrease in blood pressure in CNS-intact rats. It is well known that bradycardia is induced by visceral manipulation during surgery in human [6,7]. And these responses have been considered as a reflex response whose efferent is the cardiac vagus nerve. Using anesthetized rats, Grundy and Davison [8] suggested that the efferent path of bradycardia to a distension of the stomach was the cardiac vagus nerve. On the other hand, Robbins and Sato [4] reported that bradycardic responses to a distension of the uterus was not influenced by a bilateral vagotomy in anesthetized rats and suggested a contribution of the cardiac sympathetic nerve. It is interesting to learn the efferent paths and the reflex center of bradycardia induced by CRD.It has been reported that the cardiovascular response to CRD stimulation in anesthetized rats was unstable and fluctuated based on the depth of anesthesia [2]. Our previous study developed an anesthetized rat model of stable depressor response induced by CRD [5]. Using this rat model in the present study, we aimed to clarify the involvement of the cardiac sympathetic and parasympathetic nerves as efferent paths in the CRD-induced reflexive bradycardia. We also examined this reflex in spinalized rats to determine whether its reflex center was in the brain and/or the spinal cord. MethodsAdult male Wistar rats weighing 330-380 g (n = 16) were housed in a climate-controlled environment with food and water ad libitum. All animal experiments ...