SUMMARY1. Intracellular Na' activity (aIa) and membrane resting potential were studied in quiescent guinea-pig atrial and papillary muscles by means of Na'-sensitive and conventional microelectrodes. The effects of the cardioactive steroid dihydroouabain (DH11) on aia, force of contraction and sarcolemmal Na', K+-ATPase activity were also investigated.2. In thirty atria and twenty-two papillary muscles, a'a amounted to 8-0 + 0 2 and 4 7 +0 3 mm, respectively (means±+S.E.M.). When both tissues were from the same animal, with the same ion-sensitive microelectrode mean a'a values of 79 + 02 and 541 + 0-5 mm (P < 0 0 1) were obtained from eight atrial and eight papillary muscles, respectively.3. Membrane resting potentials (Em) were significantly (P < 0-001) more negative in the papillary muscles (-83 5 + 0 7 mV; n = 8) than in the atrium (-7841 + 0 5 mV; n = 8). Deviation of Em from EK (determined by K+-sensitive microelectrodes) was 30 + ±0)2 mV in ventricular (P < 0 05) and 641 + 0 3 mV in atrial preparations (/' < 0 05).4. Inhibition of the Na+ pump by DHO increased aia of the atrium within 10 min by 06+01 (n = 7). 13+01 (n = 5) and 32+02 mm (n = 5) at 5,10 and 30jCtM, respectively. In the papillary muscle, 10 yUM DHO was without effect while a'a rose by 1-0+0-1 (n = 5) and 2-9+02 mm (n = 6) at 30 and 120 /aM DHO.5. Consistent with the a'a measurements, the potency of DHO to increase force of the isometric contraction was three times higher in atrium than in papillary muscle (stimulation frequency 0 2 Hz).6. Hydrolytic activity of sarcolemmal Na+,K+-ATPase isolated from atria amounted to only one third of that detected in ventricles (0-07 +001, n = 6, versus 0-2+0±-01 /mol phosphate released min-(g tissue)-', n = 5). The inhibitory potencies of DHO on sarcolemmal Na+,K+-ATPase preparations were found to be identical in the enzymes from either tissue.
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