O. Z. Roy

Seasonal spontaneous torpor in the white-footed mouse, Peromyscus leucopus

Abstract 1. 1. In winter spontaneous torpor occured only in mice that had been acclimated to cold weather the preceding fall. 2. 2. Daily spontaneous torpor was very frequent from late fall to early spring. Minimum thoracic temperatures averaged 17·3°C and minimum heart rate was 60 beats/min. 3. 3. All mice became torpid when fasted in late spring and summer. Fasting-induced torpor differed from spontaneous torpor in a slower rate of arousal, higher minimum thoracic temperature and irregular heart rhythm.

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.

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.

60 Hz ventricular fibrillation thresholds for large-surface-area electrodes

The paper describes a series of animal experiments in which large-surface-area disk electrodes were used to study the current density required for ventricular fibrillation. The electrical currents were introduced to the heart both by applying the electrodes directly to the heart and by applying the electrodes to the surface of the chest near the heart. The electrode areas studied ranged from 1000 to 30 000 mm2. The results show that, for large-area electrodes, fibrillation thresholds are determined by current density. The thresholds approach a constant value of 3·5 μA mm−2

Measurement of regional cardiac fibrillation thresholds.

The paper describes the measurement of 60 Hz cardiac fibrillation thresholds for five different regions on the heart. Both bipolar and unipolar measurements indicate that the most sensitive portion of the heart to 60 Hz current flow is the apex, and that fibrillation thresholds can vary by a factor of more than 2 to 1 depending on where the current is introduced.

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.

Effect of therapeutic level ultrasound on visual evoked potentials in the hypoxic cat

The effect of 1.0 MHz ultrasound at an intensity of 1.0 W/cm2 SATA on brain function of anaesthetized cats was assessed using Visual Evoked Potentials (VEPs). Ultrasound alone did not significantly modify the averaged VEP signals. However, acute hypoxia depressed the VEP response. When the brain was exposed to ultrasound during the hypoxic episode, the VEP response remained normal. Raising brain temperature by whole body heating could not mimic the beneficial effect of ultrasound on the VEP response during hypoxia. This suggests that therapeutic ultrasound may have a protective effect on hypoxic tissues and may have clinical applications.

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.