A.J. Mortimer

The effect of therapeutic ultrasound on calcium uptake in fibroblasts

The effect of therapeutic ultrasound on calcium uptake by embryonic chick 3T3 fibroblasts has been studied using calcium-45 radiotracer techniques. Cells were treated while suspended in culture medium at 36 degrees C at intensities from 0.25 W/cm2 SPPA to 1.5 W/cm2 SPPA (1 MHz, pulsed 2 ms on:8 ms off) and for exposure times from 1 min to 20 min. Ultrasound treatment was found to increase calcium uptake for SPPA intensities from 0.5 to 1.0 W/cm2 SPPA, with a maximum increase of 18% after a 5 min exposure. Calcium uptake also increased with increasing exposure time. Measurements performed up to 20 min after treatment showed that the cell was able to reduce this calcium influx, indicating that the membrane did not suffer irreparable damage as a result of the ultrasound exposure.

A cavitation and free radical dosimeter for ultrasound

A simple and sensitive method is described for measuring the number of hydroxyl radicals (.OH) produced in aqueous solution by high intensity ultrasound (US). The method is based on the conversion of nonfluorescent terephthalic acid (TA) to highly fluorescent hydroxyterephthalate (HTA). The spectrofluorimeter is calibrated by use of a TA solution which has been exposed to a reference dose of cobalt-60. This allows fluorescence readings produced by ultrasound to be read directly in equivalent centiGrays (cGy). The limit of detection for OH is about 1.3 X 10(-9) M (equivalent to an exposure of about 0.5 cGy of cobalt-60). The threshold for transient cavitation (TC) in an air-saturated aqueous solution was found at 0.7 watts per cm2. Cavitation was consistently produced when TA was exposed in a soft vinyl chamber and only when the chamber was being rotated about an axis which was perpendicular to the US beam. The use of TA as a quantitative indicator of transient cavitation should be of value to the study of ultrasonic biological effects and dosimetry.

Ultrasound-enhanced diffusion through isolated frog skin

The effect of ultrasound on the transport of oxygen across excised frog abdominal skin has been studied. Samples were mounted in an exposure chamber in which the Ringers solution on one side was saturated with oxygen while the other side of the skin had a low initial oxygen concentration. They were treated with ultrasound at 1, 1.5 and 2 W cm-2 SATA c.w., respectively, and increases in the rate of oxygen transport were observed at all intensities. These increases ranged from 38 +/- 4% at 1 W cm-2 to 55 +/- 8% at 2 W cm-2. Variation in the pulse lengths from 25 to 200 ms and a constant average intensity did not affect the rate of transport significantly provided that the temporal intensity was constant. Since the peak acoustic pressure within the pulse increased with decreasing pulse length and increasing acoustic pressure increases the probability of cavitation occurring, the mechanism responsible for this phenomenon is probably not cavitation.

Ultrasound intensity and contractile characteristics of rat isolated papillary muscle

The effect of graded intensities of continuous wave ultrasound on the contractile performance of isolated papillary muscle of rat was tested. Under isometric conditions rat left ventricular papillary muscles (n = 48) were electrically stimulated to contract at rates of 30, 60, 120 and 240 beats per minute. Muscles were perfused with a Tyrode solution at 30 degrees C under normoxic conditions. Ultrasound at intensities of 0.25, 0.50, 1.0 and 2.0 W/cm2 spatial average temporal average (SATA) at 963 kHz was applied to the muscles while recording muscle contractile characteristics. The analog data were digitized and stored on disk for analysis by computer. This revealed a significant (p less than 0.001) increase in peak developed force (F), peak rate of force development (+dF/dt) and peak rate of myocardial relaxation (-dF/dt) that was linearly related to ultrasound intensity. The muscles were more sensitive to ultrasound at 240 contractions per minute. Resting force was significantly decreased by ultrasound. Although bath temperature increased according to the ultrasound intensity, control studies in papillary muscles (n = 24) on the correlation between contractile parameters and temperature revealed that bulk heating could not account for the positive inotropic action with ultrasound. These data confirm the inotropic effect of continuous wave ultrasound on myocardial tissue and point to the possibility of applying this phenomenon therapeutically.

Enhancement of contractility in rat isolated papillary muscle with therapeutic ultrasound

Abstract In an earlier report from our laboratory [ 8 ], we described some changes in the mechanical activity of rat isolated papillary muscles exposed to therapeutic ultrasound. With ultrasound intensity at 1.0 W/cm 2 (1MHz) we noted a significant depression of resting or diastolic force without a concomitant alteration of the developed force. We attributed this change to nonthermal effects of the irradiation since temperature compensated controls did not show the same effect. Those experiments were performed at a temperature of 26°C and a stimulation rate of 3 pulses/min. In the present experiments, we describe the effects of therapeutic ultrasound on rat isolated papillary muscles stimulated to contract at rates between 6 and 360/min at a temperature of 30°C under both normal and hypoxic-acidotic conditions. In all cases, we induced a significant increase in the developed force of muscles exposed to the ultrasound for a 5 min period.

Effects of short-duration transients on cardiac rhythm

Cardiac stimulation thresholds of short-duration large-amplitude electrical transients were studied. An isolated rabbit heart model was used and transients were applied directly to the heart through electrodes of 1 mm2 and 1 cm2 surface area. A variety of oscillatory waveforms and pulse configurations were studied and indicated that, for transients shorter than 100 μs, stimulation thresholds approach a constant charge-transfer density of 3·4 μC cm−2.

System for the measurement of the effects of ultrasound on membrane electrical and mechanical properties of the myocardium

A system has been developed to measure the effects of ultrasound on membrane electrical properties and muscle contractile parameters. A specially designed muscle bath was constructed. Measurement instrumentation for glass microelectrodes and an isometric force transducer were interfaced to a Gould transient recorder and a PDP 11/03 computer. Electrical and contractile measurements on electrically stimulated muscle were made simultaneously with resolution sufficient to observe changes in the rate of rise of the action potential. At present the system is used to study the effects of ultrasound on rat left ventricular papillary muscle.

Effect of brief ultrasound exposure on post-tetanic potentiation in cardiac muscle

The effect of a short 4 second ultrasound application (1.0 W/cm2 SATA at 963 KHz) on the post-tetanic-potentiation of isolated isometrically contracting rat papillary muscle has been evaluated. Post tetanic-potentiation was produced in hypoxic isolated papillary muscle by interrupting the control stimulation rate of 6/minute with 10 stimulating pulses at 0.2 sec intervals for one cycle. Ultrasound application varied from one second prior to the stimulating train to a period covering the entire train. Ultrasound application just preceding and impinging upon the pulse train, enhanced post-tetanic-potentiation contractions. However, delay in ultrasound application and covering the stimulating pulse train, reduced post-tetanic-potentiation contractions. These data suggest that a window exists for the effects of ultrasound on contracting myocardium which may be used to probe critical events in the cardiac cycle.

The biological significance of ultrasonically induced cavitation

There is now considerable evidence that levels of ultrasound employed clinically can produce cavitation in vitro and, in certain circumstances, in vivo. Transient and prolonged cavitation is potentially damaging, destroying cells locally and inducing chemical changes through free radical production. Although not necessarily lethal, the effects of free radical activity may modify cell activity, particularly if chemical changes are produced in the components of the cell membranes. In contrast, acoustic cavitation of the stable type can have effects which may be of advantage when ultrasound is used as a therapeutic agent, although their production should be avoided in embryonic and fetal tissue; these effects include the enhancement of protein synthesis and cell motility, and could result in the stimulation of repair phenomena by ultrasound. The conditions under which cavitation occurs must be determined so that it can be avoided where advisable and employed where appropriate. [Work supported in part by the ...

The effects of ultrasound on the electrophysiology of cardiac muscle

Using a system developed at the National Research Council of Canada the effect of ultrasound on cardiac muscle electrophysiology and on contractile properties of the myocardium has been studied. Preliminary results have been obtained from seven rat papillary muscles. Ultrasound led to a decrease in resting tension of the papillary muscles which paralleled a cellular hyperpolarization of the resting potential of 3 to 5 mV. In addition ultrasound led to an increase in the overshoot of the action potential of 26 mV. Observed changes in overshoot may be explained due to temperature changes, however changes in resting potential cannot.