Role of nonglycosidic inotropic agents: indications, ethics, and limitations

Nonglycosidic inotropic agents have been used for the short-term management of low output states and hypotension complicating acute myocardial infarction for several years. Without adequate reperfusion of the ischemic myocardium, inotropic agents are seldom effective in producing sustained hemodynamic responses. Furthermore, the potential exists for enhancement of ischemia and extension of myocardial necrosis. Thus, inotropic and vasopressors therapy should be regarded as temporary supportive treatment in patients with acute coronary syndrome and should be discontinued as soon as feasible. Parenteral sympathomimetic agents, usually dobutamine, and phosphodiesterase inhibitors, usually milrinone, are used for the management of exacerbations of chronic systolic heart failure. Although hemodynamics, and occasionally clinical status, improve, such therapy is associated with increased mortality and can potentially hasten a patient's demise. Nonparenteral sympathomimetics, such as ibopamine, phosphodiesterase-III inhibitors, such as milrinone and enoximone, calcium-sensitizing agents, such as pimobendan, and other novel inotropic agents, such as vesnarinone, all increase mortality of patients with chronic heart failure. Furthermore, newer noninotropic agents, such as B-natriuretic peptide, have been introduced for treatment of decompensated heart failure. New nonpharmacologic devices, such as biventricular pacing, are available for the treatment of advanced heart failure. Thus, indications for the use of presently available nonglycosidic inotropic agents are limited and should be considered only for short-term therapy or when no other treatment is available.

The actions of digitalis leaf preparations and of cardiac glycosides on the isolated right ventricle of the guinea pig

A method is described for recording kymographically the systolic contraction and the resting length of the electrically stimulated, isolated right ventricle of the guinea pig. Two samples of Digitalis purpurea leaf and a sample of Digitalis lanata have been assayed in terms of the Third International Standard of D. purpurea using metameters dependent on changes in the systolic contractions and resting length of the muscle and the time elapsing before the changes occur. In a similar manner digitoxin and ouabain have been assayed in terms of digoxin. The results are compared with those obtained by slow intravenous infusion of the preparations into guinea pigs. The potencies derived from the two methods are in close agreement in the case of the digitalis leaf preparations. Digitoxin, however, is more active than digoxin on the ventricle preparation irrespective of the metameter used and less active by the slow infusion method, while ouabain has the same activity as digoxin on the ventricle and twice the activity by the infusion method. The results for ouabain on the ventricle using the different metameters, are homogenous, those for digitoxin are not. There is little difference between the general actions of the three glycosides on the ventricle but each glycoside is capable of producing different effects on systolic contraction and resting length depending on the dose employed. The possible mechanisms responsible for these effects are discussed.

Effect of strophanthidin on fluxes of potassium in rabbit kidney slices

The effect of the cardiotonic steroid strophanthidin on the K content and K42 exchange rates of rabbit kidney slices has been determined. Strophanthidin has been shown to decrease the influx of K into renal tubule cells, thereby accounting for an observed reduction in the K content of slices in vitro and a diminished rate of secretion of K in vivo. It is probable that the inhibitor also decreases the rate constant for efflux of K from the tubule cells. However, the latter conclusion requires qualification since the thickness and structure of kidney slices complicate the interpretation of isotope exchange data, particularly in this case with respect to the analysis of changes in K efflux.

Chemical structure and pharmacological activity of some derivatives of digitoxigenin and digoxigenin

A series of derivatives of digitoxigenin and digoxigenin were prepared and tested for toxicity in the cat and the guinea-pig and on the isolated heart of the 48-hr chick embryo, and for inotropic activity on the cat isolated papillary muscle and the guinea-pig Langendorff heart. The order of relative potency of the compounds remained the same whether they were tested for toxicity or for positive inotropic activity. There are three molecular centres in the cardiac aglycone that are linked closely with cardiac activity. These are: (a) an OH at carbon-3 which can be combined as a glycoside, thus enhancing activity, or esterified or oxidized, producing compounds of lower activity; the maximum intensity of the inotropic response was reduced in the less potent compounds; (b) a 14-beta-OH associated with a cis C-D ring junction, alteration of which abolished activity; (c) an unsaturated cyclobutenolide ring which cannot be reduced without a great decrease in activity.

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.

OBSERVATIONS ON THE USE OF THE HEN AND RABBIT ISOLATED AURICLES FOR THE DETERMINATION OF INOTROPIC POTENCY

The electrically driven, isolated auricles from the rabbit and the hen have been used to compare the inotropic potency of a bufodiene aglycone with ouabain. The results from the hen auricle permit conventional statistical treatment and show a good degree of reproducibility. Results of inotropic potencies and toxicity determinations on the two drugs are discussed.

[Receptor function stability of cerebral Na+,K+-ATPase under conditions of modification of their enzymatic activity]

Rat and bovine Na+, K(+)-ATP-ase and isozymes have been studied for their sensitivity to ouabain to evaluate the conformational stability of the receptor for cardioactive steroid and its interconnection with intramembrane organization of catalytic polypeptide under modification of native functional conformation of membrane-bound enzyme by treatment with agents known to affect the membrane integrity (such as SDS, heat, phospholipase A2). The higher sensitivity of alpha-isoform of Na, K-ATP-ase activity to detergent as compared to alpha + under their irreversible inactivation that is due to its preferential sollubilization from the membrane is discovered. The existence of differences between isozymes in the hydrophobic protein-lipid interaction has been supposed. The greater sensitivity of alpha-isozyme to disorganization of the enzyme native phospholipid environment with phospholipase A2 has been also established. It is shown that Na+, K(+)-ATP-ase receptor function of ouabain-sensitive and ouabain-resistant enzyme types is much more conservative in comparison with hydrolytic and resistant to possible disorders of protein-lipid interactions.

Malignant hyperthermia and the general dentist: current recommendations

Malignant hyperthermia is a potentially fatal disease that may be triggered by the administration of specific drugs or by stress. Although most often associated with general anesthesia, stress can be a significant stimulus and is therefore of concern to the general dentist. The decision as to how and where to treat these patients is complicated by conflicting recommendations from various sources. The aim of this article is to propose a protocol for the dentist to use in the treatment of patients with a history of malignant hyperthermia. The dentist must assess the patient for both their susceptibility to a crisis and the degree of stress of the planned procedure. For non-stressful treatment, it is reasonable to assume that the general dental practitioner can treat these patients in his office. For stressful treatment, advanced preparation is advised.

New hydrogen-bond potentials for use in determining energetically favorable binding sites on molecules of known structure

An empirical energy function designed to calculate the interaction energy of a chemical probe group, such as a carbonyl oxygen or an amine nitrogen atom, with a target molecule has been developed. This function is used to determine the sites where ligands, such as drugs, may bind to a chosen target molecule which may be a protein, a nucleic acid, a polysaccharide, or a small organic molecule. The energy function is composed of a Lennard-Jones, an electrostatic and a hydrogen-bonding term. The latter is dependent on the length and orientation of the hydrogen bond and also on the chemical nature of the hydrogen-bonding atoms. These terms have been formulated by fitting to experimental observations of hydrogen bonds in crystal structures. In the calculations, thermal motion of the hydrogen-bonding hydrogen atoms and lone-pair electrons may be taken into account. For example, in a alcoholic hydroxyl group, the hydrogen may rotate around the C-O bond at the observed tetrahedral angle. In a histidine residue, a hydrogen atom may be bonded to either of the two imidazole nitrogens and movement of this hydrogen will cause a redistribution of charge which is dependent on the nature of the probe group and the surrounding environment. The shape of some of the energy functions is demonstrated on molecules of pharmacological interest.