Relationship of cardiac muscle tension to Na+,K+-adenosine triphosphatase activity after chronic digitoxin administration in cats.

To obtain a better understanding of the mechanism of action of the cardiac glycosides, we examined inotropic and biochemical effects of digitoxin in myocardium from cats chronically exposed to the drug. The mechanical function of papillary muscles was tested isometrically and left ventricular tissue was analyzed for Na+,K+-dependent adenosine triphosphatase ATPase activity. Muscles from control cat hearts developed tension at 2.

A comparison of the influence of La3+, D600 and gentamicin on frequency-force relationships in isolated myocardium.

The capability of heart muscle to respond mechanically to changes in frequency of stimulation was studied in the absence and in the presence of La3+, D600 and gentamicin using left atrial preparations of guniea pigs. L3+ (0.5 x 10(-3) M) and gentamicin (1 x 10(-3) M) depressed the basal contractile state at 1.

A sensitive radioenzymatic assay for catechol drugs.

This assay measures picogram quantities of catechol drugs and endogenous catecholamines in body tissues and fluids. The catechols are converted to their 3H-O-methyl metabolites during incubation with 3H-S-adenosylmethionine then separated by solvent extraction and thin-layer chromatography. Most drugs containing the catechol structure can be radiolabeled and separated from norepinephrine and epinephrine by this technique to provide simultaneous measurement of endogenous and exogenously administered catechols.

Inotropic responsiveness of atrial myocardium bathed in Tris- or bicarbonate-buffered solutions.

Atrial muscle of guinea pigs was used to study the inotropic influences of substituting a tris(hydroxymethyl)-aminomethane-buffered solution aerated with 100% O2 for a bicarbonate-buffered solution aerated with 95% O2-5% CO2 under otherwise equivalent in vitro conditions. Basal values of isometric contractile tension and its first derivative (dT/dt) were smaller and time to peak tension and time to 90% relaxation were longer in Tris- than in bicarbonate-bathed muscles. Both groups responded similarly to changes in stimulation frequently (0.