Membrane currents underlying the modified electrical activity of guinea-pig ventricular myocytes exposed to hyperosmotic solution.
1. Guinea-pig ventricular myocytes were superfused with hyperosmotic (sucrose) Tyrode solution (1.2-2.8 times (T) normal osmolality) for up to 40 min. Action potentials were recorded with microelectrodes, and membrane currents with the perforated- or ruptured-patch technique. 2. Hyperosmotic treatment for 20 min shrunk cell volume and hyperpolarized the membrane. Moderate (1.2-1.5 T) treatment caused biphasic changes in action potential configuration (rapid minor shortening quickly followed by lengthening to a stable 110% control duration). Severe (2.2-2.8 T) treatment caused triphasic changes (marked early shortening, strong rebound lengthening and subsequent pronounced shortening). At peak lengthening (6-10 min) action potentials (165% control duration) had a hump near -30 mV and slowed terminal repolarization. 3. In accordance with previous studies, hyperosmotic solution inhibited the delayed rectifier K+ current, and enhanced the outward Na(+)-Ca2+ exchange current (INaCa) at plateau potentials. A novel finding was that hyperosmolality reduced the amplitude of L-type Ca2+ current (ICa,L) and slowed its rate of inactivation. Experiments on myocytes loaded with indo-1 suggest that the reduction in ICa,L is due to a rapid elevation of [Ca2+]i. 4. When impaled myocytes were preloaded with EGTA, severe hyperosmotic treatment induced a rapid monotonic shortening of the action potential to a stable 20% of control duration. Addition of external K+ quickly nulled the hyperpolarization and slowly lengthened the action potential. 5. The results suggest that modified electrical activity in osmotically shrunken myocytes is primarily caused by increases in [K+]i, [Na+]i and [Ca2+]i: (i) elevated [K+]i hyperpolarizes the membrane (which may contribute to increased [Na+]i); (ii) elevated [Na+.]i shortens all phases of the action potential (increased outward-directed INaCa); and (iii) elevated [Ca2+]i has antagonistic plateau shortening (inhibition of inward ICa,L) and plateau lengthening (reduced outward INaCa) influences, as well as a strong subplateau lengthening effect (enhanced inward INaCa).
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Department of Physiology and Biophysics, Dalhousie University, Halifax, Nova Scotia, Canada.
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McDonald
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D000200:Action Potentials / Q000502:physiology; D000818:Animals; D004594:Electrophysiology; D005456:Fluorescent Dyes; D006168:Guinea Pigs; D006321:Heart / Q000294:innervation / Q000502:physiology*; D006352:Heart Ventricles / Q000166:cytology / Q000294:innervation; D006982:Hypertonic Solutions; D066298:In Vitro Techniques; D008297:Male; D008564:Membrane Potentials / Q000502:physiology; D009206:Myocardium / Q000166:cytology; D018408:Patch-Clamp Techniques; D016276:Ventricular Function
D005456:Fluorescent Dyes; D006982:Hypertonic Solutions