Intracellular sodium activity and its regulation in guinea‐pig atrial myocardium.

Abstract

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.

Differential Na+–K+‐ATPase activity in rat lemniscal and non ‐ lemniscal auditory Thalami

“…While the mechanism of differential Differential Na¤-K¤-ATPase expression J. Physiol. 502.2 pump regulation in the heart appears to be associated with an enhanced protein synthesis (Wang et al 1993), our data showed that in MGB neurones the cell size and the membrane pump density is an important factor. This is supported by the fact that despite the difference in membrane pump signals, the cytoplasmic pools of pump signals in the two cell populations are comparable.…”

“…If it can be assumed that both ventral and dorsal cell types express primarily the á3 isoform and that they have identical pump stoichiometry and turnover rate, then the density of pump molecules apparently becomes an important factor affecting the amplitude of the pump current. The membrane density of the Na¤-K¤-ATPase varies widely among different cell types and even in a given cell the membrane distribution of pump molecules can be highly heterogenous (Allen et al 1989;Cramb et al 1989;McGrail et al 1991;Ikenouchi, Zhao, McMillan, Hammond & Barry, 1993;Wang et al 1993). An example of such functional differential distribution has been described recently in guinea-pig heart where the atrial and ventricular myocytes are reported to contain different amounts of pump molecules and, as a result, exhibited different contractile force in response to ouabain (Wang et al 1993).…”

“…For instance, during the differentiation of skeletal muscles, there is a large increase in the number of membrane ouabain binding sites or the level of Na¤-K¤-ATPase and concurrently a significant hyperpolarization of membrane potential (for review see Clausen, 1986). Wang et al (1993) recently reported that myocardiac cells in the bovine atrium may have significantly fewer pump molecules than myocardiac cells in the ventricle. As a result, the two muscle groups exhibit distinct patterns of contractile force in response to ouabain inhibition.…”

“…The Na¤-K¤-ATPase is subject to both intra-and extracellular (via hormonal stimulation) modulation and protein phosphorylation Lingrel et al 1990;McDonough & Farley, 1993;Feschenko & Sweadner, 1995). One of the internal regulatory mechanisms, as revealed recently in heart and skeletal muscles and epithelial tissues, relies on the capacity of the cell to control the synthesis of pump protein and the quantity of pump in the plasma membrane (Mircheff, Bradley, Clausen, 1986;Yiu & Lambert, 1989;Wang, Schmied, Ebner & Korth, 1993). For instance, during the differentiation of skeletal muscles, there is a large increase in the number of membrane ouabain binding sites or the level of Na¤-K¤-ATPase and concurrently a significant hyperpolarization of membrane potential (for review see Clausen, 1986).…”

The electrogenic effects of Na+–K+‐ATPase in rat auditory thalamus

“…The electrogenic activity of the Na¤-K¤-ATPase may play an important role in setting the mode of synaptic transmission in sensory thalamus. (Wang, Schmied, Ebner & Korth, 1993). The lack of a clear pump electrogenic effect in these examples is, however, partly due to the large cell volume and their low input resistance (Thomas, 1972;Lauger, 1991).…”