Potassium: potassium exchange catalysed by the sodium pump in human red cells

1. When red cells were so depleted of Na that Na:K exchange had almost ceased, the ouabain-sensitive K efflux seen in K-containing media was accompanied by an almost equal ouabain-sensitive K influx.2. This suggests that the Na pump in these cells was carrying out a one-for-one K:K exchange across the erythrocyte membrane.3. 30-40% of the (42)K efflux from resealed ghosts was sensitive to ouabain when the ghosts contained 1 mM-ATP, 2 mM orthophosphate, 10 mM-K and less than 1 mM-Na, and the suspending medium contained 10 mM-K and 0-Na, choline being the predominant cation.4. In resealed ghosts, the rate of K:K exchange saturated as internal K was increased, and was half-maximal at about 10 mM-K.5. When internal ATP was maintained with a phosphocreatine:creatine phosphokinase regenerating system, K:K exchange saturated as internal ATP was increased, and was half-maximal at about 100 muM-ATP.6. The rate of K:K exchange did not depend on whether the ADP concentration was roughly the same as the ATP concentration or very much less, suggesting that ADP did not affect the rate of K:K exchange.7. GTP, ITP and UTP were unable to substitute for ATP in supporting K:K exchange. CTP was a poor substitute.8. There was no evidence to support the hypothesis that K:K exchange is accompanied by a ouabain-sensitive hydrolysis of ATP.9. Internal Na was a strong inhibitor of ouabain-sensitive K efflux from ghosts containing 9 mM-K. 4 mM-Na was sufficient to produce 90% inhibition.10. The rate of K:K exchange depended on the orthophosphate concentration inside the ghosts (confirming Glynn, Lew & Lüthi, 1970). The curve obtained suggested that the rate was half-maximal at about 1.7 mM orthophosphate.11. These experiments suggested that inhibition by internal K is an important factor affecting the Na efflux from intact red cells. Experiments measuring Na:K exchange as a function of internal Na in low-K ghosts supported this hypothesis.12. The significance of these findings is discussed.

Apollo 16 bio energetic considerations

During the manned lunar mission of Apollo 16, a study was undertaken to obtain more definitive information on human caloric requirements of prolonged exposure to weightless flight. Observations were made of immediate pre- and postflight body mass, total body water and exchangeable potassium. Continuous inflight recording of nutrient intake was achieved and partial metabolic excretory data were obtained. During weightlessness, the three crew members lost body water and adipose tissue. Caloric requirements calculated from these deficits proved to be different from those estimated prior to flight on the basis of body weight and age. The problem of predicting the metabolic cost of spaceflight is discussed.

Potassium-43 myocardial perfusion scanning for the noninvasive evaluation of patients with false-positive exercise tests

Twelve patients with false-positive ECG exercise tests presented with ischemic ECG responses (greater than one mm ST segment depression) during graded maximal treadmill exercise testing in the absence of exercise-induced chest pain or clinical coronary heart disease. Coronary arteriography, left ventriculography and hemodynamic evaluation revealed no significant abnormality in these patients. Transmyocardial lactate analysis during atrial pacing revealed a normal extraction (mean 21%) during pacing stress in five patients.

Nine of 12 patients were evaluated with exercise potassium-43 myocardial perfusion imaging whereby the radioactive tracer was administered intravenously during exercise at a time when abnormal ECG changes were present. Myocardial images in these nine patients all showed a normal homogeneous pattern of radioisotope distribution. These results are in direct contrast to findings in patients with coronary heart disease in whom reproducibly demonstrable abnormal regions of decreased potassium-43 accumulation have been noted in myocardial images obtained following injection of the radioactive tracer during exercise. Potassium-43 exercise myocardial perfusion scanning thus appears to be an accurate noninvasive method of assessing patients with suspected false-positive exercise tests and provides a means of increasing the specificity of exercise testing.

Altered ion transport in vascular smooth muscle from spontaneously hypertensive rats. Influences of aldosterone, norepinephrine, and angiotensin

The interaction of vascular electrolytes and early spontaneous hypertension was studied in the rat aorta. Chemical composition (H 2 O, Na, K, Ca, Mg, Cl, collagen, and elastin), extracellular space, and cell water content were little changed. Only uronic acid and hexosamine contents were significantly elevated in the spontaneously hypertensive rat. Approximately 37% of the aortic weight was cellular. Functional changes in ion transport were observed in smooth muscle from hypertensive rats; the muscle exhibited decreased ability to accumulate K and extrude Na and increased turnover of 42 K (0.0165 ± 0.0009 vs. 0.0086 ± 0.0002 min -1 ) and 36 C1 (0.162 ± 0.011 vs. 0.118 ± 0.003 min -1 ). Spontaneously hypertensive rats maintained increased 42 K exchange after adrenalectomy and reserpinization. The bioregulants, aldosterone, norepinephrine, and angiotensin had important actions on ion exchange. After adrenalectomy, aldosterone therapy reduced 42 K exchange toward intact levels. Norepinephrine increased the rate of 42 K exchange with the dose-response relation having a lower median effective dose (ED 50 ) for spontaneously hypertensive rats (10 -9 g/ml) than it did for normal Wistar rats (2 x 10 -9 g/ml). Angiotensin also increased 42 K exchange with similar dose-response relations for both groups. I concluded that functional alterations observed in spontaneously hypertensive rats probably resulted from primary changes in ion transport by vascular smooth muscle rather than from secondary effects of altered regulatory systems. The decreased selectivity to K over Na and the increased turnover of ions could lead to increased reactivity to norepinephrine through effects on membrane potentials.

Ventricular conduction delay and arrhythmias during regional hyperkalemia in the dog. Electrical and myocardial ion alterations

Coronary arterial potassium (K + ) infusion induces RS-T elevation and ventricular arrhythmias. To explore electrical and metabolic events in this model, which simulates ischemia-induced injury currents and rhythm disturbances, isotonic KCl was infused at 0.5-2.0 ml/min into the left anterior descending or the left circumflex coronary artery of 37 intact anesthetized dogs. Conduction was monitored with His and left bundle branch recording catheters and with bipolar electrodes at endocardial and epicardial levels in the perfused zone. RS-T segments rose progressively in the perfused region despite normal coronary blood flow ( 85 Kr) some dogs progressed rapidly to multifocal ventricular extrasystoles and ventricular fibrillation (group 1), but others continued for 20 minutes without developing fibrillation (group 2). His-Q and bundle branch conduction times were not altered by regional perfusion, but the QRS complex was locally prolonged. Ventricular extrasystoles were frequently coupled and left bundle branch depolarizations never preceded them, suggesting origin within the myocardium. Intramural electrodes showed increasing delay in the perfused region; epicardial depression was uniformly more profound and preceded significant arrhythmias, often with 2:1 or higher intramural block and slowing sufficient to allow reentry. Epicardial activation preceding endocardial activation was observed during extrasystoles in the perfused zone, further indicating origin within the myocardium. These phenomena were associated with tissue concentrations of K + in the outer wall significantly higher than those in the inner wall; and sodium was also significantly elevated. Therefore, it was postulated that reentry from depressed epicardial layers into more normal surrounding tissue constitutes a possible basis for the arrythmias in this model.

Regional distribution of diffusible tracers and carbonized microspheres in the left ventricle of isolated dog hearts

Microspheres of different sizes, ¹²⁵I-labeled antipyrine (I-Ap), and ⁴²KCl or ⁸⁶RbCl were injected into the aortic inflow of isolated, Langendorff, perfused, nonworking dogs hearts at blood flows of 1.3–4.8 ml/min g⁻¹. After 15 seconds to 5 minutes, the left ventricle was sectioned into about 300 ordered pieces, and the amount of each tracer was determined. For all tracers, the relative density of deposition was generally higher in the endocardial region, except in one heart in which the aortic pressure and the total coronary flow were low. The deposition of ⁴²K and that of I-Ap were essentially similar in three hearts over a large range of regional variation. This finding suggests either that both tracers were distributed in proportion to flow or that a small diminution in relative density of deposition of ⁴²K in high-flow regions due to lower transcapillary extraction was quantitatively similar to a decrease in the residual fraction of I-Ap in these same regions due to faster washout in the first 15–30 seconds after injection. Large microspheres were deposited preferentially in regions of high flow, exaggerating the apparent heterogeneity of regional flows. The distribution of the smaller microspheres was closer to that for I-Ap or ⁴²K.