A dual-pipette technique that permits rapid internal dialysis and membrane potential measurement in voltage-clamped cardiomyocytes

Proteolytic modification of swelling-activated Cl- current in LNCaP prostate cancer epithelial cells

The effects of intracellular application of trypsin on the Cl- current induced by hypotonic cell swelling (I(Cl,swell)) in human prostate cancer epithelial cells (LNCaP) was studied using the patch-clamp technique. In cells predialyzed with 1 mg/mL trypsin, I(Cl,swell)) developed and diminished in response to the application and withdrawal of hypotonic solution about three times faster than that in control cells. In trypsin-infused cells, I(Cl,swell)) also had about twofold higher current density and displayed considerably slowed voltage-dependent inactivation, which was quite pronounced in control cells at potentials above +60 mV. Trypsin-induced modification of I(Cl,swell)) could be prevented by coinfusion of 10 mg/mL soybean trypsin inhibitor, suggesting that proteolytic cleavage of essential intracellular structural domains of the I(Cl,swell))-carrying volume-regulated anion channel (VRAC) was responsible for this functional modification. The effect of trypsin was not dependent on the presence of intracellular ATP. We conclude that VRACs, similarly to voltage-gated Na+, K+, and Cl- channels, possess intracellular inactivation domain(s) subjected to proteolytic cleavage that may function in conformity with the classical "ball-and-chain" inactivation model.

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).

Second messenger-controlled membrane conductance in locust (Locusta migratoria) olfactory neurons

The membrane conductance of olfactory neurons of Locusta migratoria was examined using the whole-cell configuration of the patch-clamp technique. Intracellular application of the second messenger inositol 1,4,5 trisphosphate (IP(3)) via a dual pipette technique elicited a clear increase in the membrane conductance. The IP(3)-induced conductance increased due to a rise in the extracellular concentration of calcium from 100&mgr;M to 4mM. Micromolar concentrations of ruthenium red partially blocked the IP(3)-induced increase in membrane conductance. Stimulating olfactory neurons with odour (hexenoic acid) resulted in an increase in the membrane conductance partially similar to that mediated IP(3). These findings suggest that stimulation with appropriate odours as well as intracellular application of IP(3) activate the same calcium-permeable ion channels in the plasma membrane of insect olfactory neurons.

Optical multisite monitoring of cell excitation phenomena in isolated cardiomyocytes

An especially designed setup which consists of an inverted fluorescence microscope, an argon ion laser and a photodiode array system permits membrane potential monitoring in isolated guinea-pig ventricular cardiomyocytes, stained with the voltage-sensitive dye di-4-ANEPPS, which responds linearly with relative fluorescence changes (delta F/F) approximately -8% per 100 mV. About a dozen measuring spots covering a single cell were simultaneously monitored with a spatial and temporal resolution of 15 microns and about 20 microseconds, respectively. In general, the rising phases of the action potentials within a single cell were highly synchronized (i.e. all upstroke velocities peaked within about 20 microseconds); however, in one cell (out of 25 examined) significant (P < 0.05) time lags exceeding the signal-dependent time resolution were also found. Experiments, simultaneously performed with our optical system and a widely used patch-clamp setup, revealed a slowed and delayed response of the clamp amplifier depending on the cell access resistance. Optical monitoring during whole-cell voltage-clamping demonstrated the influence of graduated series resistance compensation. When field stimulation was used, our results clearly demonstrated the spatially dependent polarization of the cell membrane during the stimulus, as well as a highly synchronized upstroke development. Slight differences in the maximum upstroke velocities within a single cell were also found and were basically in agreement with mathematical models.

Alkali cation permeability and caesium blockade of cromakalim-activated current in guinea-pig ventricular myocytes

1. The sensitivity of cromakalim-activated current (Icrom) to manipulations of extracellular cationic composition was examined in whole-cell voltage clamp recordings from freshly-dispersed, adult guinea-pig ventricular myocytes. In bathing media with different concentrations of K+ (1, 2.5, 5.4 and 12 mM) the Icrom reversal potential (Erev) varied in strict correspondance with the K+ equilibrium potential and inward Icrom amplitude was proportional to the external K+ concentration. 2. Replacement of 12mM K+ with 12mM Rb+ induced a slight positive shift of Erev indicating that PRb+/PK+ = 1.06. K+ replacement with 12mM Cs+ reduced or abolished inward Icrom and produced a negative shift of Erev by at least 50 mV; an upper limit of PCs+/PK+ was fixed at 0.18. 3. Addition of Rb+ (1-30 mM) to 2.5 mM K(+)-containing medium produced a concentration-dependent increase in inward Icrom and positive shift of Erev suggesting that K+ and Rb+ have similar permeabilities and conductivities and do not interfere with each other in the channel. 4. CS+ (0.01-30 mM), added to medium containing 12 mM Rb+, induced a potent, voltage-dependent inhibition of inwardly rectifying current (IK1; IC50 = 0.2-3 mM). Voltage-dependent inhibition of inward Icrom was observed only at considerably higher CS+ concentrations (IC50 = 4-30 mM). Extracellular Rb+ and CS+ did not substantially alter the amplitude of outward Icrom. 5. The results support the contention that the ATP-sensitive K+ channel is the primary target of cromakalim action in ventricular myocytes.