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 2.5 +/- 0.7 g/mm2; muscles from cats that received subcutaneous digitoxin--100 micrograms/kg on day 1, followed by 40 micrograms/kg/day for 4 days (group A), and 75 micrograms/kg on day 1, followed by 25 micrograms/kg/day for 9 days (group B)--developed significantly greater (p less than 0.05) tension of 4.8 +/- 0.3 and 3.6 +/- 0.6 g/mm2, respectively. Further, in vitro maximal responsiveness to digitoxin was greater in the muscles from digitalized groups than in controls (p less than 0.05): Muscles from control cats had a maximal response to in vitro addition of digitoxin of 3.5 +/- 0.1 g/mm2; muscles from cats in group A reached 4.9 +/- 0.3 g/mm2, and those from group B, 4.5 +/- 0.7 g/mm2. Specific activity of microsomal Na+,K+-ATPase from hearts of digitalized groups A and B was inhibited by 50-70% (p less than 0.01). Developed tension, specific Na+,K+-ATPase activity, and in vitro maximal responsiveness to digitoxin in a third group (C) of cats receiving the least daily digitoxin (75 micrograms/kg on day 1, followed by 15 micrograms/kg/day for 29 days) were not different from controls. Mean plasma digitoxin concentrations were 33, 16, and 3 ng/ml in groups A, B, and C, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)