Effects of external tetraethylammonium ions and quinine on delayed rectifying K+ channels in mouse pancreatic beta-cells

Specificity of tetraethylammonium and quinine for three K channels in insulin-secreting cells

The effects of tetraethylammonium (TEA) and quinine on Ca-activated [K(Ca)], ATP-sensitive [K(ATP)]K channels and delayed-rectifier K current [K(dr)] have been studied in cultured insulin-secreting HIT cells using the patch-clamp technique. K(Ca) and K(ATP) channels were identified in excised, outside/out patches using physiological solutions and had unitary conductances of 60.8 +/- 1.3 pS (n = 31) and 15.4 +/- 0.3 pS (n = 40), respectively. Macroscopic K(dr) current (peak current = 607 +/- 100 pA at +50 mV, n = 14) were recorded in the presence of 100 microM cadmium and 0.5 microM tetrodotoxin. Tetraethylammonium (TEA) blocked all three channel types but was more effective on K(Ca) channels (EC50 = 0.15 mM) than on K(ATP) channels (EC50 = 15 mM) or K(dr) currents (EC50 = 3 mM). Quinine also blocked all three currents but was less effective on K(Ca) channels (EC50 = 0.3 mM) while equally effective against K(ATP) channels and K(dr) currents (EC50 = 0.025 mM). TEA blocked K(Ca) and K(ATP) channels by reducing their single-channel conductances and decreasing the probability of K(ATP) channel opening. Quinine blocked K(Ca) channels by reducing the single-channel conductance, but blocked K(ATP) channels by reducing the probability of channel opening. Reinterpretation of previous microelectrode studies in light of these findings suggest that, (i) only K(ATP) channels are active in low glucose, (ii) both K(Ca) and K(dr) channels may assist Ca-spike repolarization, and (iii) K(Ca) channels play no role in forming the burst pattern of Ca spiking in the B cell.

Sequence and functional expression in Xenopus oocytes of a human insulinoma and islet potassium channel

Regulation of insulin secretion involves the coordinated control of ion channels in the beta-cell membrane. We have isolated and characterized cDNA and genomic clones encoding a voltage-dependent K+ channel isoform expressed in human islets and in a human insulinoma. This K+ channel isoform, designated hPCN1, with a deduced amino acid sequence of 613 residues (Mr = 67,097), is related to the Shaker family of Drosophila K+ channels. hPCN1 is homologous to two other human K+ channel isoforms we have isolated, hPCN2 and hPCN3, with 55% and 65% amino acid sequence identity, respectively. The electrophysiological characteristics of hPCN1 were determined after microinjection of synthetic RNA into Xenopus oocytes. Two-microelectrode voltage-clamp recordings of oocytes injected with hPCN1 RNA revealed a voltage-dependent outward K+ current that inactivated slowly with time. Outward currents were inhibited by 4-aminopyridine with a Ki less than 0.10 mM and were relatively insensitive to tetraethylammonium ion or Ba2+. A delayed rectifier K+ channel such as hPCN1 could restore the resting membrane potential of beta cells after depolarization and thereby contribute to the regulation of insulin secretion.

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.

Delayed rectifying and calcium-activated K+ channels and their significance for action potential repolarization in mouse pancreatic beta-cells

The contribution of Ca2(+)-activated and delayed rectifying K+ channels to the voltage-dependent outward current involved in spike repolarization in mouse pancreatic beta-cells (Rorsman, P., and G. Trube. 1986. J. Physiol. 374:531-550) was assessed using patch-clamp techniques. A Ca2(+)-dependent component could be identified by its rapid inactivation and sensitivity to the Ca2+ channel blocker Cd2+. This current showed the same voltage dependence as the voltage-activated (Cd2(+)-sensitive) Ca2+ current and contributed 10-20% to the total beta-cell delayed outward current. The single-channel events underlying the Ca2(+)-activated component were investigated in cell-attached patches. Increase of [Ca2+]i invariably induced a dramatic increase in the open state probability of a Ca2(+)-activated K+ channel. This channel had a single-channel conductance of 70 pS [( K+]o = 5.6 mM). The Ca2(+)-independent outward current (constituting greater than 80% of the total) reflected the activation of an 8 pS [( K+]o = 5.6 mM; [K+]i = 155 mM) K+ channel. This channel was the only type observed to be associated with action potentials in cell-attached patches. It is suggested that in mouse beta-cells spike repolarization results mainly from the opening of the 8-pS delayed rectifying K+ channel.

Block of ATP-regulated and Ca2(+)-activated K+ channels in mouse pancreatic beta-cells by external tetraethylammonium and quinine

1. The whole-cell and outside-out patch configurations of the patch-clamp technique were used to investigate the effects of extracellular tetraethylammonium ions (TEA+) and quinine on both Ca2(+)-activated and ATP-regulated K+ channels in mouse pancreatic beta-cells. 2. The Ca2(+)-activated K+ channel has a single-channel K+ permeability of 4.7 x 10(-13) cm3 s-1 when recorded with physiological ionic gradients. This value decreased to 2.9 x 10(-13) cm3 s-1 after addition of 0.3 mM-TEA+. 3. Two exponentials with time constants of 0.2 and 4.7 ms were required to describe the distribution of the channel openings suggesting that the Ca2(+)-activated K+ channel has at least two open states. The fast and slow components comprised 16 and 84% of the total number of openings respectively. 4. TEA+ caused a concentration-dependent decrease in the single-channel amplitude and open probability of the Ca2(+)-activated K+ channel. A Kd for the reduction in the mean current of 0.14 mM was observed. The stoichiometry was approximately 1:1. 5. Quinine blocked the Ca2(+)-activated K+ channel in a concentration-dependent manner. Half-maximal block was observed at 0.10 mM and binding was 1:1. Inhibition by 20 microM-quinine was not associated with a decrease in channel amplitude but markedly reduced the lifetime of the channel openings. Two exponentials, with time constants of 0.5 and 1.3 ms, were required to describe the channel openings. The rapid component contained 55% of the events. 6. TEA+ reduced the single-channel amplitude of the ATP-regulated K+ channel in a concentration-dependent manner. Kd for the block was 22 mM and the binding approximately 1:1. The block was not associated with changes in the open probability or channel kinetics. Two exponentials were required to describe the distribution of the open times. The time constants for the fast and slow components were approximately 2 and approximately 20 ms respectively. The rapid component accounted for approximately 35% of the events. 7. Quinine (10-20 microM) almost abolished activity of the ATP-regulated K+ channels. Inhibition was characterized by slow onset and reversibility but not associated with a change in the appearance of the single-channel events. Quinine-induced block could not be reversed by diazoxide. 8. We conclude that TEA+ produces rapid block of both Ca2(+)-activated and ATP-regulated K+ channels.(ABSTRACT TRUNCATED AT 400 WORDS)