Dietary phosphate-dependent growth is not mediated by changes in plasma phosphate concentration.

The present study was undertaken in order to assess the role of dietary phosphate in growth. A diet deficient in phosphate led to a suppression of growth in juvenile rats. The phenomenon is similar to that described for diets deficient in other essential single components such as Mg, Zn or K. However, unlike the other constituents, dietary phosphate restriction affected the growth rate much more than it altered the serum phosphate concentration; addition of phosphate to the drinking water of rats fed a low-phosphate diet restored the growth rate without a concomitant change in serum phosphate concentration. The suppression of growth rate by the deletion of phosphate was associated with a delayed decrease in food intake. This finding implies that the variation in appetite was secondary to the change in growth. The increase in body weight following phosphate supplementation was associated with a concomitant increment in food intake. The phosphate-dependent growth was, however, evident also in rats that were pair-fed with those that were not supplied with phosphate. It is concluded that dietary phosphate-dependent growth is not mediated by changes in phosphate concentrations in the extracellular fluid. It is plausible that signals arising from receptors for phosphate in the digestive system constitute part of the growth control apparatus in rats.

Long-chain fatty acids activate calcium channels in ventricular myocytes.

Nonesterified fatty acids accumulate at sites of tissue injury and necrosis. In cardiac tissue the concentrations of oleic acid, arachidonic acid, leukotrienes, and other fatty acids increase greatly during ischemia due to receptor or nonreceptor-mediated activation of phospholipases and/or diminished reacylation. In ischemic myocardium, the time course of increase in fatty acids and tissue calcium closely parallels irreversible cardiac damage. We postulated that fatty acids released from membrane phospholipids may be involved in the increase of intracellular calcium. We report here that low concentrations (3-30 microM) of each long-chain unsaturated (oleic, linoleic, linolenic, and arachidonic) and saturated (palmitic, stearic, and arachidic) fatty acid tested induced multifold increases in voltage-dependent calcium currents (ICa) in cardiac myocytes. In contrast, neither short-chain fatty acids (less than 12 carbons) or fatty acid esters (oleic and palmitic methyl esters) had any effect on ICa, indicating that activation of calcium channels depended on chain length and required a free carboxyl group. Inhibition of protein kinases C and A, G proteins, eicosanoid production, or nonenzymatic oxidation did not block the fatty acid-induced increase in ICa. Thus, long-chain fatty acids appear to directly activate ICa, possibly by acting at some lipid sites near the channels or directly on the channel protein itself. We suggest that the combined effects of fatty acids released during ischemia on ICa may contribute to ischemia-induced pathogenic events on the heart that involve calcium, such as arrhythmias, conduction disturbances, and myocardial damage due to cytotoxic calcium overload.

Clinical suppression of refractory ventricular tachycardia with oral bretylium not predicted by electrophysiologic drug testing.

We report the findings in a patient in whom intravenous bretylium was the only effective agent to suppress refractory ventricular tachycardia and ventricular fibrillation. After attempts to switch the patient to amiodarone and bethanidine (an oral analogue of bretylium) caused proarrhythmic effects, he was successfully converted to oral therapy with bretylium. Electrophysiologic testing was not predictive of the clinical response from oral bretylium. To our knowledge, this is the first report of a proarrhythmic effect from bethanadine and it suggests a divergence in the actions of various class 3 antiarrhythmic agents.

Intracellular bretylium blocks Na+ and K+ currents in heart cells.

The effects of intracellular and extracellular application of bretylium tosylate on outward potassium current and fast inward Na+ current have been studied in single ventricular cells of chick embryo (18-day-old) using the whole-cell voltage clamp technique. Extracellular superfusion with bretylium (3 x 10(-4) M) decreased the K+ current (IK) whereas the fast INa remained unaffected. Introducing bretylium intracellularly also decreased IK, but rapidly inhibited the fast INa. The addition of 3 x 10(-4) M bretylium to the superfusion medium in the presence of intracellular bretylium further decreased IK and caused the fast inward sodium current (INa) to recover. The decrease of IK by bretylium may account for the antifibrillatory properties of this drug in heart.

Bethanidine increased Na+ and Ca2+ currents and caused a positive inotropic effect in heart cells.

The effects of bethanidine sulphate, a pharmacological analog of the cardiac antibrillatory drug, bretylium tosylate, were studied on action potentials (APs) and K+, Na+, and Ca2+ currents of single cultured embryonic chick heart cells using the whole-cell current clamp and voltage clamp technique. Extracellular application of bethanidine (3 X 10(-4) M) increased the overshoot and the duration of the APs and greatly decreased the outward K+ current (IK) and potentiated the inward fast Na+ currents (INa) and the inward slow calcium current (ICa). However, intracellular introduction of bethanidine (10(-4) M) blocked INa. In isolated atria of rat, bethanidine increased the force of contraction in a dose-dependent manner. These findings suggest that when applied extracellularly, bethanidine exerts a potentiating effect on the myocardial fast Na+ current and slow Ca2+ current and an inhibitory effect of IK. The positive inotropic effect of bethanidine could be due, at least in part, to an increase of Ca2+ influx via the slow Ca2+ channel and the Na-Ca exchange. It is suggested that the decrease of IK by bethanidine may account for its antifibrillatory action.

Potassium channel blockade: A mechanism for suppressing ventricular fibrillation.

The suppression of ventricular fibrillation by antidysrhythmic drugs is well correlated with their ability to block potassium channels in nerve and cardiac membranes. Blockade of potassium channels reduces electrical inhomogeneities in action potential and conduction parameters that lead to ventricular fibrillation. These actions tend to effectively decrease the electrical size of the heart, which suggests a mechanism for antifibrillatory drug action. The receptor sites for antifibrillatory drug action (IK blockade) appear to be on the outside of the cardiac membrane whereas receptors for antiarrhythmic drug action (INa blockade) appear to be on the inside of the cardiac membrane.

Antiarrhythmic and electrophysiologic actions of bethanidine sulfate in primary ventricular fibrillation or life-threatening ventricular tachycardia.

Antiarrhythmic and electrophysiologic actions of bethanidine sulfate, a chemical analog of bretylium tosylate, were studied using programmed cardiac electrical stimulation in 14 survivors of out-of-hospital cardiac arrest unassociated with acute myocardial infarction. Before bethanidine sulfate was administered sustained ventricular tachyarrhythmias (VT) were inducible in 11 patients and reproducible nonsustained VT was induced in 3 patients. Bethanidine sulfate shortened sinus cycle length and absolute and relative ventricular refractory periods measured during sinus rhythm, but did not alter ventricular effective refractory period measured during ventricular pacing. Bethanidine sulfate prevented inducible VT in 8 patients (57%), increased the number of extrastimuli needed to induce VT in 2 patients, and was ineffective in 4 patients. In contrast, in only 1 of 26 trials with other conventional and investigational antiarrhythmic drugs in these patients was VT prevented. Orthostatic hypotension was a prominent side effect of bethanidine sulfate therapy, but could be reversed in most patients by concomitant administration of protriptyline. Five patients in whom bethanidine sulfate was effective in the laboratory have been treated chronically (400 to 600 mg 4 times daily), and all are alive at 3 to 40 months. In the remaining 9 patients, 8 were treated empirically because no drug was effective in the laboratory and 1 was treated with quinidine, which appeared to be protective during testing. Four of these 9 patients, including the patient treated with quinidine, died suddenly during follow-up. Thus, although bethanidine sulfate therapy is difficult to initiate because of orthostatic hypotensive side effects, it may be useful in treating patients at high risk of recurrent cardiac arrest.

Bretylium opens mucosal amiloride-sensitive sodium channels.

Addition of the quanternary ammonium compound, bretylium, to the outer surface of a frog skin leads to an increase in the potential difference and in the short circuit current across the skin. Bretylium does not have any effect when applied to the inside face of the frog skin. The effect of bretylium is dependent upon the presence of sodium ions in the outer medium; it is depressed when sodium is replaced by choline or potassium but not when lithium substitutes for sodium. The bretylium effect is blocked by the specific sodium channel blocker, amiloride. It is proposed that bretylium opens mucosal, amiloride-sensitive sodium channels.

Antiarrhythmic, antifibrillatory, and hemodynamic actions of bethanidine sulfate: an orally effective analog of bretylium for suppression of ventricular tachyarrhythmias.

Bethanidine sulfate is a chemical and pharmacologic analog of bretylium tosylate that has virtually identical antifibrillatory and inotropic actions on the heart. Bretylium is the only drug approved by the Food and Drug Administration specifically for the "prophylaxis and treatment of ventricular fibrillation." Unlike bretylium, which is poorly absorbed from the gut and limited to parenteral use, oral bethanidine is absorbed rapidly. Bethanidine was given to 23 patients with recurrent multiple drug refractory ventricular tachycardia and fibrillation. Eighteen patients (78%) had complete suppression of spontaneous or electrophysiologically inducible tachyarrhythmias; 3 were improved and 2 had no benefit. In 6 of a 9 patient subgroup studied by programmed electrophysiologic drug testing, bethanidine completely prevented previously inducible ventricular tachyarrhythmias at the maximal stimulus tested (including 4 extrastimuli and burst-pacing at 10 times threshold). Cardiac output measured in 6 patients 1 to 2 hours after bethanidine was increased in 4, unchanged in 2, and decreased in 1. Ten patients on long-term therapy with bethanidine and protriptylene (to prevent orthostatic hypotension) have been free of arrhythmias from 2 to 26 (average 13) months.

Suppression of ventricular fibrillation and positive inotropic action of bethanidine sulfate, a chemical analog of bretylium tosylate that is well absorbed orally.

Bethanidine sulfate is a closely related chemical analog of bretylium that has virtually identical pharmacologic and antifibrillatory actions on the ventricle. Unlike bretylium, which is very poorly absorbed from the gut, bethanidine is rapidly and effectively absorbed after oral administration. Bethanidine increased ventricular fibrillation threshold in the normal dog heart from an average control value of 28.5 mA to an average peak value of 66.5 mA, an increase of 150 percent. In the infarcted heart, bethanidine increased ventricular fibrillation threshold from an average postinfarction level of 14.4 mA to an average peak value of 55.3 mA, an increase of 327 percent. Like bretylium, the antifibrillatory action of bethanidine was manifested by the appearance of nonsustained ventricular fibrillation when superthreshold shocks induced episodes of ventricular fibrillation lasting from 2 to 120 seconds and converting spontaneously to sinus rhythm. In contrast, the untreated dog heart must always be defibrillated electrically. The onset of antifibrillatory action began as early as 2 minutes after intravenous administration and 15 to 30 minutes after oral administration; peak action occurred in approximately 60 minutes. Bethanidine had prolonged positive inotropic action in the isolated heart as reflected by an increase in cardiac output and blood pressure lasting up to 60 minutes. Bethanidine lowered coronary vascular resistance and increased coronary blood flow. The oral efficacy and rapid onset of action gives bethanidine a potential role in the prevention of out-of-hospital ventricular fibrillation.

Scanning transmission electron microscope studies of deep-frozen unfixed muscle correlated with spatial localization of intracellular elements by fluorescent x-ray analysis.

Thin sections of deep-frozen unfixed muscle were studied in a scanning electron microscope modified for transmission imaging and equipped with a "cryostage" for vacuum compatibility of hydrated tissue. With an energy-dispersive x-ray analysis system, intracellular atomic species in the scan beam path were identified by their fluorescent x-rays and spatially localized in correlation with the electron optical image of the microstructure. Marked differences are noted between the ultrastructure of deep-frozen hydrated muscle and that of fixed dehydrated muscle. In frozen muscle, myofibrils appear to be composed of previously undescribed longitudinal structures between 400-1000 A wide ("macromyofilaments"). The usual myofilaments, mitochondria, and sarcoplasmic reticulum were not seen unless the tissue was "fixed" before examination. Fluorescent x-ray analysis of the spatial location of constituent elements clearly identified all elements heavier than Na. Intracellular Cl was relatively higher than expected.

Coronary blood flow, oxygen delivery rate and cardiac performance.

1. Studies have been made on the isolated blood-perfused heart of dogs in which isometric tension development at various settings of resting tension (RT) was measured at various levels of O(2) delivery rates controlled by altering (a) coronary blood flow (CBF), (b) O(2) capacity or (c) O(2) saturation of the perfusate. Measurements were also made of O(2) consumption and vascular perfusion resistance.2. The capacity of the left ventricle to develop tension at any given setting of resting tension was found to be directly correlated with changes in O(2) delivery rate using any of the above three methods of altering the latter.3. The slopes of the curves relating resting tension to developed tension are positively correlated with total O(2) delivery to the heart.4. The O(2)-dependent metabolic effect upon tension production was found to be slow in development, in contrast to the Frank-Starling effect, which reached full development in the first heart beat after a change in resting length and tension.5. The O(2) consumption of the isometrically contracting heart is strongly correlated with the O(2) delivery rate at all particular values of resting tension and related developed tension.6. The metabolic state of the myocardium as determined by the rate of O(2) delivery within physiological ranges is (a) a direct major determinant of the tension-producing capacity of the heart muscle and (b) determines the magnitude of adaptation via the Frank-Starling mechanism.