Thermally-mediated ultrasound-induced contraction of equine muscular arteries in vitro and an investigation of the associated cellular mechanisms.

We have previously shown that MHz frequency ultrasound causes contraction of the carotid artery in vitro. We now extend this investigation to equine mesenteric arteries and investigate the cellular mechanisms. In vitro exposure of the large lateral cecal mesenteric artery to 4-min periods of 3.2 MHz continuous wave ultrasound at acoustic powers up to 145 mW induced reversible repeatable contraction. The magnitude of the response was linearly dependent on acoustic power and, at 145 mW, the mean increase in wall stress was 0.020 ± 0.017 mN/mm(2) (n = 34). These results are consistent with our previous study and the effect was hypothesised to be thermally mediated. A 2°C temperature rise produced an increase in intracellular calcium, measured by Fluo-4 fluorescence. Inhibition of the inward-rectifier potassium ion channel with BaCl(2) (4 µM) increased the response to ultrasound by 55% ± 49%, indicating a similar electrophysiologic basis to the response to mild hyperthermia. In small mesenteric arteries (0.5-1.0 mm diameter) mounted in a perfusion myograph, neither ultrasound exposure nor heating produced measureable vasoconstriction or a rise in intracellular calcium and we conclude that temperature-sensitive channels are absent or inactive in these small vessels. It, therefore, appears that response of blood vessels to ultrasound depends not only on the thermal properties of the vessels and surrounding tissues but also on the electrophysiology of the smooth muscle cells.

Ultrasound-induced contraction of the carotid artery in vitro.

Ultrasound is known to produce a range of nonlethal responses in cells and tissues. Frequencies in the kilohertz ultrasound range have been shown to produce relaxation in large arteries. The present work explores the effects of insonation at MHz frequencies, representative of those used diagnostically and therapeutically, in an in vitro preparation of the carotid artery. Fresh 12.7 mm wide rings of equine common carotid artery obtained from the abattoir were mounted in a purpose-made myograph. They were immersed in a bath of Krebs-Ringer buffer at 37 degrees C and were positioned at the focus of an ultrasound beam from a weakly focused 3.2 MHz source. Continuous wave insonation produced contraction. The tension increased rapidly over the first 2 min, followed by a slower increase for the duration of the exposure up to 15 min. At a power of 145 mW a maximum contractile stress of 0.04 +/- 0.03 mN/mm(2) (mean +/- SD, n = 77) was measured, which was approximately 4% of the maximum wall stress generated by noradrenaline (0.1 mM). The magnitude of the response was weakly dependent on power in the range 72-145 mW and was not significantly different for pulsed and continuous wave stimulation where time averaged power was constant. The response was unaffected by mechanical removal of the endothelium. The ultrasound beam generated insufficient radiation force to produce a measurable effect and streaming at the vessel surface was very small compared with flow rates known to produce physiologic effects. The temperature rise at the beam focus was approximately 0.3 degrees C and we hypothesise that this contributes to the observed response, probably through changes in ion channel activity in smooth muscle cell membranes. (E-mail: e.martin@exeter.ac.uk).