Effects of hyperbaric pressure and temperature on atria from ectotherm animals (Rana pipiens and Anguilla anguilla).

1. Spontaneously beating atria from frogs (R. pipiens) and eels (A. anguilla) were compressed hydraulically to 10 MPa. Effects on beating frequency and twitch tension were studied. 2. At low temperatures (8-10 degrees C) compression to 10 MPa caused a slowing of the beat frequency. No effects were noted at higher temperatures (16-24 degrees C). Twitch tension was decreased by pressure at low temperatures and increased at high temperatures. 3. Differences were noted between preparations from cold and warm acclimatized frogs, and from silver and yellow eels, respectively. 4. The effect of temperature acclimatization on pressure and temperature sensitivity is discussed in relation to data on cardiac phospholipid fatty acid composition.

Effects of hydrostatic pressure and inert gases on twitch tension.

Effect of pressure and inert gases on the twitch tension (Tmax) was measured on electrically stimulated and spontaneously beating rat atria. In stimulated preparations, pressurization to 10 MPa increased Tmax by 20-60% depending on the stimulating frequency (60-240 beats/min). The introduction of 5 MPa N2 or 5 MPa H2 at 10 MPa hydrostatic pressure decreased the Tmax by 17 +/- 6% and 13 +/- 6%, respectively. Gas effect did not depend on the stimulating frequency. Nitrous oxide (0.15 and 0.45 MPa) decreased Tmax both at "surface" and at 10 MPa. Nitrous oxide effect was slightly potentiated at pressure. In spontaneously beating preparations, compression to 10 and 15 MPa decreased beating frequency (BF) by 24 +/- 10% and 31 +/- 8% and increased Tmax by 60 +/- 35% and 105 +/- 33%, respectively. The tension increase is partly due to the direct pressure effect and partly due to the negative force-frequency relation in the rat atria. Introduction of inert gas increased BF and decreased Tmax. The potency of the gases was in the same order for both variables: He less than H2 less than N2.

Effects of hydrostatic pressure, H2, N2, and He, on beating frequency of rat atria.

Hydrostatic compression to 15 MPa caused a drop in spontaneous beating frequency (BF) of isolated rat atria kept in tris solution at 37 degrees C by 30.6 +/- 7.2%. Introduction of superfusing solutions equilibrated with hydrogen (PH2: 4.9, 9, and 14 MPa, respectively), increased the BF in proportion to the hydrogen content. A hydrogen partial pressure equal to the hydrostatic pressure was calculated to reduce the bradycardia by 52.0 +/- 19.5%. Effects of nitrogen (PN2: 5 and 14 MPa) and helium (PHe: 13 and 14 MPa) were also tested. Nitrogen was found to be 1.7-2 times and helium 0.2 times as effective as hydrogen in reducing the bradycardia. Preparations compressed at 27 degrees C exhibited a more pronounced bradycardia than those kept at 37 degrees C, but 5 MPa N2 and 9 MPa H2 reversed the bradycardia to the same extent at 27 degrees C as at 37 degrees C. Tests with 4 MPa H2 showed the effect on BF to be similar, whether the gas was added during an intermediate stop in the compression (4.6 MPa) or at 10 MPa pressure. An additional hydrostatic pressure increase from 10 to 12.5 MPa eliminated the BF increase of 4 MPa hydrogen added at 10 MPa. The findings are discussed in view of the possible use of hydrogen as a breathing gas in deep sea diving.

Effects of dietary lipids on frequency and force in atrial muscle at 10 MPa.

Spontaneously beating atrial preparations, from rats fed different lipid diets, were compressed to 10 MPa. The following observations were made: Different lipid diets altered the ratio of omega-6/omega-3 polyunsaturated fatty acids of the cardiac phospholipids. Beating frequency and twitch tension at surface pressure was unaffected by the diets. Compression to 10 MPa caused a decrease in spontaneous beating frequency and an increased twitch tension in all preparations. The decrease in beating frequency was inversely related to the omega-6/omega-3 ratio. Pressure induced increase in twitch tension was not affected by the diets. N2O dissolved in the tissue bath solution partly counteracted the pressure-induced changes.

Pressure equilibration of the middle ear during ascent.

The capacity to equilibrate the middle ear with the ambient pressure depends on different factors. During descent when the clearing is active the technique is the most important factor. During ascent when the clearing is passive the status of the mucosal membranes lining the eustachian tube is believed to be of major importance. Horizontal position in air and head-up immersion in water to the neck have in earlier investigations shown to decrease the passive clearing capacity compared to vertical, head-up position in air. It was found in this study that a change from head-up position in water to prone in water did not change in clearing capacity, while head-down position in water gave a significant decrease of the clearing capacity. The results are discussed in terms of venous pressure in the neck veins in the different positions.

Effects of high hydrostatic pressure on rat atrial muscle.

Muscle preparations from rat atria were hydraulically compressed in circulating Tris-buffered solution kept at 37 degrees C. Spontaneously beating preparations decreased their beating frequency with 37.3 +/- 13.5 beats/min (22.3% +/- 6.7%, P less than 0.001) and increased their force with 2.3 +/- 1.1 mN (48.6% +/- 17.5%, P less than 0.001) when they were compressed to 10 MPa (100 atm). Decompression gave values not significantly different from precompression control values. The increase in force could in part be explained by the hyperbaric bradycardia and negative force-frequency relation. The remaining force increase seemed to be an effect of the increase in hydrostatic pressure. Changes in action potential duration and Ca2+ availability for the contractile machinery are discussed as possible mechanisms. Electrically driven preparations increased their contraction force at compression if the stimulus strength was at least 175% of the threshold. At lower stimulus levels just above threshold and at higher frequencies, inconsistent results were obtained at pressure.

Hyperbaric chamber for evaluating hydrostatic pressure effects on tissues and cells.

A chamber system is described for the study of pure hydrostatic pressure effects on tissues and cells. The small chamber has an internal volume of 7.6 liters and is rated for working pressures up to 400 ATA. Sliding doors at each end permit easy access and quick sealing. A cam-driven pump provides constant flow of physiological solution to the tissue bath containing the preparation. Connections to the pump allow a variety of test solutions to be used in the course of an experiment. The tissue bath is designed to prevent chamber gas from diffusing in to the perfusate, thus allowing for pure hydrostatic compression of the bath contents. The bath is coupled to a motorized stage to facilitate placement of recording devices once the bath is placed inside the chamber. Temperature is controlled within 0.05 degrees C of set point by thermoelectric modules coupled to a feedback amplifier. This system has been used for electrical and mechanical studies of cardiac muscle, but its versatility makes it suitable for a wide range of other biomedical applications.

Spontaneous activity of rat portal vein at normal and elevated hydrostatic pressure.

The effects of high hydrostatic pressure on the spontaneous contractile activity of isolated rat portal veins were studied. During compression, an increase of activity was seen, whereas stable elevated hydrostatic pressure gave a decrease of both frequency and time-integrated force. Decompression further reduced the activity, but all changes were reversible upon return to control pressure. During sustained high pressure the frequency of contractions was reduced by 15.9% at 25 atm, 26.4% at 50 atm, and 45.8% at 100 atm. The corresponding reductions in integrated active force were 13.7%, 16.7%, and 40.7%, respectively. Contractions caused by electrical stimulation of nerve endings left in the preparation were reduced by 44.1%, and potassium contractures were reduced by only 15.3% at 100 atm. It is concluded that inhibition of activity in rat portal vein at high hydrostatic pressure is due in part to effects on the smooth muscle membrane.

Rate of pressure change and hyperbaric bradycardia in the mouse sinus node.

Sinus node preparations from mice were hydraulically compressed at 10, 100, and 500 atm x min-1 in Tyrode's solution at 27 degrees C. At the highest compression rate, both a delay and a more pronounced beating frequency response to pressure was seen. The delayed reaction is ascribed to either adiabatic effects or a time-delay in conformational changes in the pacemaker cell membrane. The potentiating effect of a high compression rate could be eliminated by autonomic blockade (atropine and practolol).

Influence of nitrous oxide, nitrogen, neon, and helium on the beating frequency of the mouse sinus node at high pressure.

The beating frequency (BF) reducing effect of 150 atm of hydrostatic pressure on mammalian cardiac pacemaker tissue (hyperbaric bradycardia) was counteracted by dissolved gas only if the gas was added after hydrostatic compression. The effect on BF seemed to be related to the narcotic potency of the gas and the effect was reversible. The gases tested were N2O, N2, Ne, and He, in decreasing order of potency. If N2O was added at a moderately raised ambient pressure prior to hydrostatic compression to 150 atm, there was no difference in the degree of hyperbaric bradycardia, compared to compression without gas. During decompression, however, experiments performed with gas showed a significantly higher gain in BF compared to experiments without gas. Autonomic blockade seemed to eliminate the difference between decompression with and without N2O. The results demonstrate that N2O, N2, and Ne, and to a small extent He, may counteract the retarding effect that increased hydrostatic pressure has on cardiac pacemaker activity. These effects on the cardiac pacemaker are similar both to the effects of increased hydrostatic pressure and of gases at elevated pressures on the central nervous system, but some important differences remain to be explained.

Heart rate and respiratory frequency in hydrostatically compressed, liquid-breathing mice.

The effects of hydraulic compression on heart rate and respiratory frequency were studied in liquid-breathing, hypothermic (17-31 degrees C) mice. Increasing the hydrostatic pressure caused a bradycardia that was first evident at 25 at. and progressed to 48% of the control heart rate at 175 at. The bradycardia was reversed, although incompletely, by decompression. Similar changes in respiratory frequency were seen. Autonomic blockage with atropine and propranolol did not change the response patterns to any major extent. Compression rate (2-6 at. x min-1) did not seem to influence the degree of heart-rate reduction. Compression caused an increase in colonic temperature, and decompression a decrease (0.5 degree C for a pressure change of 100 at.). These temperature changes could be ascribed partly to adiabatic heating and cooling of the body tissues as revealed by similar changes in dead animals and partly to increased metabolic heat generation in connection with compression-induced convulsions. The temperature changes, although partly accounting for the hysteresis in the heart-rate changes during compression/decompression, were not responsible for the major effects. It was concluded that high pressure causes bradycardia by a direct action on cardiac-pacemaker cells.