Regulatory Volume Decrease of Cardiac Myocytes Induced by β-Adrenergic Activation of the Cl− Channel in Guinea Pig

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Abstract
A new method was developed to automatically measure the thickness of a single ventricular myocyte of guinea-pig heart. A fine marker was attached on the cell's upper surface and changes in its vertical position were measured by focusing it under the microscope. When the osmolarity of the bath solution was varied, the cell thickness reached a new steady level without any obvious regulatory volume change within the period of observation up to 15 min. The cell thickness was 7.8 ± 0.2 μm (n = 94) in the control Tyrode solution and was varied to 130.4 ± 3.1% (n = 10), 119.1 ± 1.1% (n = 50), 87.2 ± 1.9% (n = 9), and 75.6 ± 3.2% (n = 5) of control at 50, 70, 130, and 200% osmolarity, respectively. The application of a Cl− channel blocker, 500 μM anthracene-9-carboxylic acid (9AC) did not modify these osmotic volume changes. We discovered that the application of isoprenaline induced a regulatory volume decrease (RVD) in cells inflated by hypotonic solutions. This isoprenaline-induced RVD was inhibited by antagonizing β-adrenergic stimulation with acetylcholine. The isoprenaline-induced RVD was mimicked by the external application of 8-bromoadenosine 3′:5′-cyclic monophosphate. The RVD was inhibited by blocking the cAMP-dependent Cl− channel (ICl, cAMP) with 9AC but was insensitive to 4, 4′-diisothiocyanostilbene-2, 2′-dissulphonate (DIDS). Taken together these data suggest an involvement of ICl, cAMP activation in the RVD. Whole cell voltage clamp experiments revealed activation of ICl, cAMP by isoprenaline under the comparable conditions. The cardiac cell volume may be regulated by the autonomic nervous activity.