Rapid resetting of human baroreflex working range: insights from sympathetic recordings during acute hypoglycaemia.

1. Human muscle nerve sympathetic activity (MSA), which is governed by baroreflexes and involved in blood pressure homeostasis, is increased during acute insulin-induced hypoglycaemia. 2. To elucidate the detailed relationship between MSA and blood pressure during hypoglycaemia, 0.15 i.u. (kg body weight)-1 regular human insulin was given intravenously to eight fasting, healthy volunteers. Microneurographic recording of MSA in the peroneal nerve was made with simultaneous monitoring of arterial blood pressure by a finger cuff (Finapres). The course of MSA and blood pressure was monitored for 30 min before and 60 min after insulin injection. In three subjects control recording without insulin injection was made for the same duration. 3. Stimulus-response regression lines were constructed by plotting diastolic blood pressure against the occurrence frequency of MSA for the period of initial rest, the period of maximal MSA outflow during hypoglycaemia and the period of glucose counter-regulation. The control experiments were analysed for the corresponding periods. 4. The stimulus-response line was stable throughout the control experiments, whereas it was shifted either to the left or to the right during hypoglycaemia, and regularly shifted to the left (i.e. towards a lower blood pressure) during glucose counter-regulation, without change of slope. 5. It is concluded that the changes in MSA during acute hypoglycaemia are not secondary to baroreflex regulation but instead are characterized by acute resetting of the baroreflex working range without change of sensitivity.