Effect of intravenous infusion of an alpha-adrenergic blocking agent on the haemodynamic changes in human masseter muscle induced by cold pressor stimulation

This study evaluated the effect of intravenous infusion of a non-selective alpha-adrenergic blocking agent on masseter muscle haemodynamics induced by 4 degrees C cold presser stimulation (CPS) of the right foot and ankle, wh ich reportedly evokes a rapidly increasing sympathetic nerve activity in hu man skeletal muscle. Nine healthy non-smoking males (mean age 23.7+/-2.1 ye ar) with no history of chronic muscle pain or migraine participated. The ha emoglobin (Hb) concentration in the right masseter was continuously recorde d by non-invasive, near-infrared spectroscopy. Heart rate and blood pressur e were also recorded. The experiment involved the following sequence: (I) a placebo (physiological saline) with a CPS trial; (2) a 30-sec maximal volu ntary clenching (MVC)-only trial; and (3) an alpha-adrenergic blocking agen t with a CPS trial. The saline and drug trials each involved continuous rec ording for 1 min before, 2 min during and 5 min after the CPS, Physiologica l saline (20 mi) or phentolamine mesylate (20 ml) were infused at the rate of 3 ml/min. This infusion was begun 15 min before baseline recording and p articipants were not aware which solution (saline or phentolamine) was bein g infused. For the MVC trial, each participant performed a 30-sec MVC of hi s jaw-closing muscles followed by a 15-min rest between each trial. The ind ividual Hb data were adjusted so that the baseline at the beginning of the experiment was equal to zero and all data were normalized as a percentage o f the individual's highest absolute Hb change seen after the MVC. The mean baseline Hb concentrations 1 min before CPS were significantly higher in th e alpha-blocker trial (83.6%) than in the placebo saline trial (P < 0.001). The change in mean Hb concentration from baseline during CPS in the alpha- blocker trial was significantly less than in the placebo trial (P = 0.006). Mean heart rate before CPS was also significantly higher in the alpha-bloc ker trial (85.2 beats/min) than in the placebo trial (69.6 beats:min) (P < 0.001). There were no significant differences in the mean systolic and dias tolic blood pressures between the placebo and alpha-blocker trials in any t ime period. The results suggest that non-selective alpha-adrenoceptor block ade increases the blood volume in the masseter muscle. This change might be due to a combination of peripheral vasodilation and an increase in cardiac output. (C) 1999 Elsevier Science Ltd. All rights reserved.