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The activity of cardiopulmonary reflexes elicited by 5-hydroxytryptamine (5-HT) and phenyldiguanide (PDG) has been examined in anaesthetised 5- and 12-week-old spontaneously hypertensive (SHR) rats. Intravenous injections of 5-HT or PDG elicited dose-dependent bradycardic and hypotensive responses. In the 5-week-old SHR rats the bradycardic and hypotensive responses to 5-HT were smaller than those measured in WKY rats. At 12 weeks, the SHR rats were less sensitive to the bradycardic actions of 5-HT, as well as PDG, although the reduction in cardiopulmonary reflex sensitivity was less pronounced than that observed in the 5-week-old SHR rats. These findings suggest that cardiopulmonary reflexes are impaired in the SHR rat although some amelioration of this phenomenon may occur as hypertension develops. |
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