Ca(2+)-blockable, poorly selective cation channels in the apical membrane of amphibian epithelia. Tetracaine blocks the UO2(2+)-insensitive pathway

Membrane currents in cultured human intestinal smooth muscle cells

Using whole-cell patch-clamp recording techniques, we have examined voltage-gated ion currents in a cultured human intestinal smooth muscle cell line (HISM). Experiments were performed at room temperature on cells after passages 16 and 17. Two major components of the whole-cell current were a tetraethylammonium-sensitive (IC50 = 9 mM), iberiotoxin-resistant, delayed rectifier K+ current and a Na+ current inhibited by tetrodotoxin (IC50 A 100 nM). No measurable inward current via voltage-gated Ca2+ channels could be detected in these cells even with 10 mM Ca2+ or Ba2+ in the external solution. No current attributable to calcium-activated K+ channels was found and no cationic current in response to muscarinic receptor activation was present. In divalent cation-free external solution two additional currents were activated: an inwardly rectifying hyperpolarization-activated current, I(HA), and a depolarization-activated current, I(DA) x I(HA) and I(DA) could be carried by several monovalent cations; the sizes of currents in descending order were: K+ > Cs+ > Na+ for I(HA) and Na+ > K+ >> Cs+ for I(DA). I(HA) was activated and deactivated instantaneously and showed no inactivation whereas I(DA) was activated, inactivated and deactivated within tens of milliseconds. These currents were inhibited by external calcium with an IC50 of 0.3 microM for I(DA) and an IC50 of 20 microM for I(HA). Cyclopiazonic acid (CPA) induced an outward, but not an inward current. SK&F 96365, a blocker of store-operated Ca2+ channels, suppressed I(DA) with a half-maximal inhibitory concentration of 9 microM but was ineffective in inhibiting I(HA) at concentrations up to 100 microM. Gd3+ and La3+ strongly suppressed I(DA) at 1 and 10 microM, respectively and were less effective in blocking I(HA) (complete inhibition required a concentration of 100 microM for both). Carbachol at 10-100 microM evoked about a 3-fold increase in I(HA) amplitude and completely abolished I(DA). We conclude that I(HA) and I(DA) are Ca2+-blockable cationic currents with different ion selectivity profiles that are carried by different channels. I(DA) shows novel voltage-dependent properties for a cationic current.

Cation channel blocked by extracellular Ca2+ in the apical membrane of the chick embryonic ectoderm

In the chick embryo (20 h incubation, gastrula stage), the apical membrane of the ectodermal cells shows a high density of a non-selective cation channel which is blocked by very low extracellular Ca2+ concentrations. Properties of this channel were studied at the single-channel level using the patch-clamp technique in the cell-attached mode. With 1 mmol/l Ca2+ in the pipette, only outward current was present and the channel conductance measured at +120 mV was 25.5 pS. In the absence of Ca2+, also inward current through the channel was observed. The conductances measured at -50 mV were 49.5 pS with Na+ as the charge carrier, 72.5 pS with K+, 49.1 pS with Cs+, and 18.5 pS with Li+. The conductance measured at +80 mV was around 23 pS in all four cases. The reversal potential was similar (around 25 mV) for all four ions, which indicates a poor selectivity of the channel. In the absence of Ca2+ and the presence of 1 mmol/l ethylenebis(oxonitrilo)tetraacetate (EGTA), the kinetics of the channel were characterized by bursts of the order of seconds. During a burst, the channel flickered between one open and one closed level. The open time was constant between -30 mV and -80 mV, while the closed time decreased with hyperpolarization. The open time varied according to the permeant ion (K+ < Na+ = Cs+ < Li+). Extracellular Ca2+ blocked the inward current in a voltage-dependent manner. The Kd values, 1 mumol/l at -30 mV and 3.2 mumol/l at -80 mV, indicate that Ca2+ ions exit the channel toward the intracellular side. A weak voltage dependency of the association rate constant suggests that the Ca(2+)-binding site is close to the outside mouth. Extracellular Ca2+ was much less efficient at blocking the outward current (Kd about 1 mmol/l at 80 mV). Tetracaine, but not uraniumdioxide, decreased the opening probability of the channel. The embryonic channel shows similarities with the Ca(2+)-blockable, poorly selective channel described in the epithelium of toad urinary bladder.

Poorly selective cation channels in apical membranes of epithelia

The apical membrane of frog skin contains two types of pathways which allow the passage of several monovalent cations in the absence of external Ca2+. Differences between the two pathways concern their open-close kinetics, selectivity, and the affinity for several blocking agents. Type S channels open and close relatively slowly, whereas type F channels display fast open-close kinetics. Both channel types allow the passage of Na+, K+, and Rb+ currents which are blocked by divalent cations and La3+ added to the extracellular side. Type F channels are permeable for Cs+ which is, however, excluded from type S channels. Shifts in open-close kinetics induced by Mg2+ occur at concentrations below 5 microM for type F channels, whereas more than a tenfold higher dose is required for the type S pathway. UO2(2+) concentrations up to 100 microM only occlude type S channels while 100 microM tetracaine selectively blocks type F channels. Apical membranes of toad urinary bladder, cultured amphibian renal epithelia (A6), and toad colon contain only type F channels. In toad bladder and A6 cells volume expansion strongly activates this pathway. Macroscopic currents carried by Ba2+ and Ca2+ could be recorded after activation of toad bladders with oxytocin and treatment of the apical surface with nanomolar concentrations of Ag+, which seems to interact with a site located at the channel interior.