Adrenaline, anti-adrenaline drugs and potassium movements in rabbits auricles

CALCIUM IN RELATION TO THE ACTIONS OF OUABAIN AND ADRENALINE ON THE HEART

Isolated pairs of rabbit auricles have been observed in media containing 6, 24 or 75 mM-potassium, with corresponding reductions in sodium concentration. In 24 mM-potassium, adrenaline restored beating and excitability, as did calcium chloride, but ouabain had no effect. In 75 mM-potassium, adrenaline had no effect; calcium chloride caused a contracture; ouabain had no direct effect, but auricles which had been beating in the presence of ouabain contracted promptly on transfer to 75 mM-potassium. Left auricles, which do not beat spontaneously, were less sensitive to calcium and to ouabain. The results showed a membrane stabilizing action of calcium and an action on muscular contraction, and suggested that cardiac glycosides acted by causing accumulation of calcium at the activator site in the tissue.

Potassium and the heart

The electrical stability of the heart is more sensitive to the extracellular than to the intracellular potassium concentration. During exercise, extracellular potassium varies rapidly. Catecholamines also modulate the plasma potassium concentration. Hypokalaemia of any cause can precipitate arrhythmias. Ischaemic myocardium loses potassium into the extracellular space within seconds and the cell becomes depolarized. The rise of the extracellular potassium ion concentration accounts for many of the early electrophysiological changes. Abrupt changes of plasma potassium concentration in normal myocardium and a high potassium concentration in ischaemic myocardium can set up electrical forces which initiate arrhythmias. The same phenomenon can account for changes on the electrocardiogram early after the cessation of an exercise test in a patient with ischaemic heart disease. Accumulation of potassium between cells in response to an increase of heart rate is a possible mechanism for false positive exercise tests and Syndrome X.

The effects of terbutaline on acid base, serum electrolytes, and glucose homeostasis during the management of preterm labor

Terbutaline (T) is a beta-adrenergic compound which is commonly employed as a tocolytic agent in preterm labor. We evaluated the metabolic and biophysical consequences of infusion of T into six pregnant women in preterm labor. Our results showed that the infusion of T led to the development of hyperglycemia, hyperinsulinemia, hyperlactacidemia, hypokalemia, a fall in serum colloid osmotic pressure and pH, a rise in maternal heart rate, and a fall in maternal blood pressure. These changes in electrolytes in the serum occurred in the absence of any significant alterations in their urinary excretion. These data suggest that the infusion of T causes hyperglycemia that results in a rise in serum insulin and lactate, which, in turn, leads to a fall in serum potassium, ionized calcium, and pH. Volume expansion as manifested by a fall in colloid osmotic pressure may contribute to the reduction in the levels of ionized and total calcium. Thus, careful monitoring of electrolytes and hydration status is warranted when beta-mimetic agents are used, specifically T, for tocolytic therapy.

Actions of adrenaline on the potassium balance of the isolated heart

1. The action of adrenaline on the K(+) balance of the isolated heart was found to depend on the ionic composition and the temperature of the perfusion fluids used.2. When the perfusion fluid contained 145 mM Na(+), 1.7 mM Ca(++) and 3-9 mM K(+), adrenaline caused the hearts to gain K(+); when the K(+) concentration was reduced to 1.2 mM, adrenaline caused a loss of K(+). Both were actions on beta receptors.3. When the Ca(++) or the Na(+) concentration in the perfusion fluid was reduced, together with a reduction in K(+), adrenaline no longer produced K(+) loss from the hearts, but produced a gain of K(+).4. When the temperature of the perfusion fluid was reduced to 25 degrees C, adrenaline still produced a gain of K(+) by hearts perfused with fluid containing 3.2 mM K(+), but did not produce a loss of K(+) from hearts perfused with fluid containing 1.2 mM K(+).