d Original Contribution ULTRASOUND-INDUCED CONTRACTION OF THE CAROTID ARTERY IN VITRO ELEANOR M. MARTIN,* FRANCIS A. DUCK, y RICHARD E. ELLIS,* and C. PETER WINLOVE* * School of Physics, University of Exeter, Stocker Road, Exeter, United Kingdom; and y Department of Physics, University of Bath, Bath, United Kingdom (Received 15 January 2009; revised 28 July 2009; in final form 26 August 2009) Abstract—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  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 5 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  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) Ó 2010 World Federation for Ultrasound in Medicine & Biology. Key Words: Artery, Vasoconstriction, Bioeffects, Ultrasound. INTRODUCTION Ultrasound is generally accepted as a safe and versatile modality for medical imaging. It is also widely used for physiotherapy and is gaining acceptance as an agent to accelerate bone healing. Extracorporeal shock-wave litho- tripsy is often the treatment of choice for renal calculi and the use of high-intensity focused ultrasound is being devel- oped in minimally invasive surgery. Efforts to assess the safety of these procedures and to understand the mecha- nisms responsible for therapeutic effects have demonstrated a number of nonlethal cellular responses to ultrasound. Early experiments showed very general effects such as plasma membrane damage (reviewed in e.g., Milowska et al. 2005) and alterations in DNA synthesis (e.g., Kondo and Yoshii 1985). Further work has shown more specific responses including altered expression of NO synthase (Hsieh 2005), alterations in calcium channel activity (Liu et al. 2006; Mortimer and Dyson 1988) and upregu- lation of extracellular matrix synthesis (Sena et al. 2005). Relatively little is known about the effects of low- power ultrasound in intact tissues; most of the observa- tions mentioned above have been made on cells in culture. The present work reports such investigations on arteries in vitro, chosen for study because the vascular system is likely to be sensitive to ultrasonic stimuli and its responses could lead directly to therapeutic or pathologic effects. The effective functioning of the circulation depends critically on changes in vascular tone which regulate local and systemic blood flow and tissue nutrition. Vascular tone is determined by a complex interplay between the endothelial cells lining the vessel wall and the smooth muscle cells of the underlying tissue that affect the response. The cellular interaction depends on a wide range of signals, including cytokines such as nitric oxide and free radicals that can be exchanged between the two cell populations, and also on mechanical and fluid mechanical forces that are transduced mainly by the endothelial cells Address correspondence to: Eleanor M. Martin, School of Physics, University of Exeter, Stocker Road, Exeter, EX1 2PB, United Kingdom. E-mail: e.martin@exeter.ac.uk 166 Ultrasound in Med. & Biol., Vol. 36, No. 1, pp. 166–172, 2010 Copyright Ó 2010 World Federation for Ultrasound in Medicine & Biology Printed in the USA. All rights reserved 0301-5629/10/$–see front matter doi:10.1016/j.ultrasmedbio.2009.08.013