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

Abstract

Eight healthy non-smoking males (mean age: 24.1±1.1 years) without any history of chronic muscle pain and migraine participated in this study. Haemoglobin (Hb) and oxygen (O 2) saturation in the right masseter muscle were continuously recorded with a non-invasive near-infrared spectroscopic device. Heart rate and blood pressure were also recorded. The experiment had three phases: a placebo drug (physiological saline) with cold-pressor trial, a 30-sec maximal voluntary clenching (MVC) trial, and a propranolol with cold-pressor trial. The saline and drug trials each involved continuous recording for 1 min before, 2 min during and 5 min after the cold-pressor stimulation (4°C). Physiological saline (20 ml) or propranolol hydrochloride (20 ml) were infused at the rate of 2 ml/min. This infusion was begun 20 min before the baseline recording and participants did not know which solution (saline or propranolol) was being infused. For the MVC trial, each participant was asked to perform a 30-sec clench of their jaw-closing muscles. There was a rest period of 15 min between each trial. The individual Hb and O 2 data were normalized so that the baseline at the beginning of the experiment was equal to zero, and the Hb and O 2 data were normalized as a percentage of the individual’s own highest absolute Hb and O 2 after and during the MVC, respectively. The results showed that the mean baseline Hb 1 min before cold-pressor stimulation was significantly lower in the β-blocker trial than in the placebo trial ( p=0.035). The mean change in Hb from baseline during cold-pressor stimulation in the β-blocker trial was also significantly less than in the placebo trial ( p=0.035). The mean Hb rebound change after the cold-pressor stimulation in the β-blocker trial was significantly higher than in the placebo trial, and no significant heart-rate differences were observed in the period after cold-pressor stimulation. Overall, the mean heart rate before and during that stimulation was significantly lower in the β-blocker trial than the placebo trial ( p<0.001). There was no significant mean blood-pressure difference between placebo and β-blocker trials at any time. These results suggest that β-adrenoceptor blocking decreases the blood volume in the resting masseter, suppresses the incremental blood-volume change during cold-pressor stimulation, and discloses a hidden vasoconstrictive effect after that stimulation.