Low Ca2+-sensitive maxi-K+ channels in human cultured fibroblasts

Charybdotoxin-sensitive K(Ca) channel is not involved in glucose-induced electrical activity in pancreatic beta-cells

The effects of charybdotoxin (CTX) on single [Ca2+]-activated potassium channel (K(Ca)) activity and whole-cell K+ currents were examined in rat and mouse pancreatic beta-cells in culture using the patch-clamp method. The effects of CTX on glucose-induced electrical activity from both cultured beta-cells and beta-cells in intact islets were compared. K(Ca) activity was very infrequent at negative patch potentials (-70 less than Vm less than 0 mV), channel activity appearing at highly depolarized Vm. K(Ca) open probability at these depolarized Vm values was insensitive to glucose (10 and 20 mM) and the metabolic uncoupler 2,4 dinitrophenol (DNP). However, DNP blocked glucose-evoked action potential firing and reversed glucose-induced inhibition of the activity of K+ channels of smaller conductance. The venom from Leiurus quinquestriatus hebreus (LQV) and highly purified CTX inhibited K(Ca) channel activity when applied to the outer aspect of the excised membrane patch. CTX (5.8 and 18 nM) inhibited channel activity by 50 and 100%, respectively. Whole-cell outward K+ currents exhibited an early transient component which was blocked by CTX, and a delayed component which was insensitive to the toxin. The individual spikes evoked by glucose, recorded in the perforated-patch modality, were not affected by CTX (20 nM). Moreover, the frequency of slow oscillations in membrane potential, the frequency of action potentials and the rate of repolarization of the action potentials recorded from pancreatic islet beta-cells in the presence of glucose were not affected by CTX. We conclude that the K(Ca) does not participate in the steady-state glucose-induced electrical activity in rodent pancreatic islets.

Identification of Ca(2+)-activated K+ channels in cells of embryonic chick osteoblast cultures

Primary cultures of embryonic chick osteoblasts consist of a heterogeneous cell population. Patch clamp measurements were done on 1- to 5-day-old osteoblasts, osteocytes, fibroblastlike cells, and cells that could not be classified on morphologic criteria. The measurements showed the omnipresence of depolarization-activated high-conductance channels in cell-attached patches. The whole-cell experiments showed an outward rectifying conductance activating at positive membrane potentials. Channels underlying the latter conductance were found to be K+ conducting in outside-out membrane patches. The activation potential of the outward rectifying K+ conductance shifted to negative membrane potentials upon increasing the intracellular Ca2+ concentration within the range of 10(-8)-10(-3.2) M. The same happened with the activation potential of the K+ channels found in outside-out patches. Finally, inside-out patch experiments directly demonstrated the dependency of the activation potential of K+ channels on Ca2+ ions. Thus the identity and main characteristics of Ca2(+)-activated K+ channels expressed by the various cell types present in chick osteoblast cultures have now been established. Decreased input resistances were found in cells of cultures more than 2 days old. This is consistent with the establishment of electrical coupling between the cells. Functions in which Ca2(+)-activated K+ channels could play a role are discussed.

A potassium channel in cultured chondrocytes

Chondrocytes, obtained from preosseous cartilage, were studied by patch clamp technique in cell-attached recording configuration, and single potassium channels were characterized at different stages of culture. After 3 days, outward currents were present, with an open probability increasing with depolarization, and the K+ channels showing a mean slope conductance of 82 pS in asymmetric and 168 pS in symmetric potassium solution. Tetraethylammonium (TEA) and quinidine blocked the channels. Cells at confluence showed similar channel activity, with conductances of 121 and 252 pS, respectively. We suggest that culture time and/or conditions may modify K+ channels or induce the expression of a new type of channels.

Temperature sensitivity of Ca currents in chick sensory neurones

We have investigated the effects of temperature on the Ca currents of chick sensory neurones. Raising the temperature from 17 to 37 degrees C, caused low-threshold (LVA, T) and high-threshold (HVA, L and N) Ca currents to show a marked amplitude increase and a drastic acceleration of their activation-inactivation gatings. Compared to HVA channels, the LVA type showed a weaker temperature sensitivity. Its average Q10 values were closer to those of other voltage-operated ion channels: 1.7 (permeability), 1.9 (activation) and 2.2 (inactivation). Alternatively, the activation kinetics and peak permeability of HVA Ca channels showed maximal Q10 values of about 5 and 2.8, respectively. HVA channel deactivation was less sensitive to temperature (Q10 1.8). Inactivation of these channels was slow and monoexponential between 17 and 22 degrees C, but faster and double exponential above 30 degrees C, uncovering a fast temperature-sensitive decaying phase. The size and rate of decay of this component decreased with increasing membrane depolarizations and persisted at holding potentials positive to -80 mV, suggesting the involvement of temperature-sensitive Ca-mediated processes in the mechanism of HVA channel inactivation. Our data are consistent with the view that heating from 17 to 37 degrees C causes both an increased probability of Ca channels to open and a drastic acceleration of their activation-inactivation kinetics.

A class of non-selective cation channels in human fibroblasts

Non-selective cation channels were detected in membrane patches of cultured human fibroblasts. The channels had a unitary conductance which ranged from 14 to 25 pS in symmetrical 130 mM NaCl and were permeable to both sodium and potassium ions. Open channel probability was dependent either on the membrane potential and the Ca2+ concentration on the intracellular side of the membrane. High Ca2+ concentrations in the millimolar range were needed to keep the channel active.