Potassium channels that are inhibited by intracellular ATP (ATP i ) were first identified in ventricular myocytes, and are referred to as ATP-sensitive K + channels (i.e. K ATP channels). Subsequently, K + channels with similar characteristics have been demonstrated in many other tissues (pancreatic β-cells, skeletal muscle, central neurones, smooth muscle). Approximately one decade ago, K ATP channels were cloned and were found to be composed of at least two subunits: an inwardly rectifying K + channel six family (Kir6.x) that forms the ion conducting pore and a modulatory sulphonylurea receptor (SUR) that accounts for several pharmacological properties. Various types of native K ATP channels have been identified in a number of visceral and vascular smooth muscles in single-channel recordings. However, little attention has been paid to the molecular properties of the subunits in K ATP channels and it is important to determine the relative expression of K ATP channel components which give rise to native K ATP channels in smooth muscle. The aim of this review is to briefly discuss the current knowledge available for K ATP channels with the main interest in the molecular basis of native K ATP channels, and to discuss their possible linkage with physiological functions in smooth muscle.

1 The effects of levcromakalim (BRL 38227) on ionic currents recorded from pig proximal urethra were investigated by use of tension measurement and patch clamp techniques (conventional whole-cell configuration, nystatin perforated patch, and cell-attached configuration).2 Levcromakalim (1 guM) caused a relaxation in the resting tone. This levcromakalim-induced relaxation was inhibited by the pretreatment with 1 gM glibenclamide. 3 The resting membrane potential recorded from single cells in current-clamp mode was -36.1+4.4mV (n=5).4 Levcromakalim induced a concentration-dependent hyperpolarization with a maximum (at > 10 gM) close to the theoretical equilibrium potential of potassium (EK). The membrane hyperpolarization caused by 1 gUM levcromakalim (24.7 + 5.8 mV, n = 4) was abolished by 1 pM glibenclamide.5 Levcromakalim (100 gM) caused an outward K current in whole-cell recordings which was unaffected by iberiotoxin (300 nM) but abolished by glibenclamide (10 gM).6 In cell-attached patches, levcromakalim activated a 43 pS K channel which was inhibited by the application of glibenclamide. 7 The metabolic poison, cyanide (CN), also activated a 43 pS K channel which was suppressed by the application of 10 gM glibenclamide. 8 These results indicate that levcromakalim and metabolic inhibition activate the same 43 pS K channel in pig proximal urethra. The resultant urethral hyperpolarization might reduce the usefulness of K channel openers in the treatment of detrusor instability, but be of value in treating outflow obstruction.

1 E ects of levcromakalim and nucleoside diphosphates (NDPs) on both membrane currents and unitary currents in pig proximal urethra were investigated by use of patch clamp techniques (conventional whole-cell con®guration, nystatin perforated patch, cell-attached con®guration and inside-out patches). 2 Levcromakalim produced a concentration-dependent outward current at a holding potential of 750 mV. The peak current amplitude showed little variation when measured by either conventional whole-cell or nystatin perforated patch con®gurations. 3 In conventional whole-cell con®guration, the levcromakalim (100 mM)-induced outward current decayed by about 90% in 18 min. In contrast, with the nystatin perforated patch, approximately 86% of the levcromakalim-induced outward current still remained after 18 min. 4 The peak amplitude of the levcromakalim (100 mM)-induced outward membrane current recorded by the conventional whole-cell con®guration was greatly reduced by inclusion of 5 mM EDTA in the pipette. The much smaller but signi®cant outward membrane current remaining was abolished by glibenclamide. 5 In conventional whole-cell recordings, inclusion of an NDP in the pipette solution induced a small outward current which slowly reached a maximal amplitude (in 2 to 10 min) and was suppressed by glibenclamide. Addition of 100 mM levcromakalim after the NDP-induced current had peaked activated a further outward current which was larger than that recorded in the absence of NDPs. Approximately 50% of this current still remained at 18 min, even when conventional whole-cell con®guration was used. 6 In the cell-attached mode in symmetrical 140 mM K + conditions, glibenclamide inhibited the 100 mM levcromakalim-activated 43 pS K + channel in a concentration-dependent manner, showing an inhibitory dissociation constant (K i ) of approximately 520 nM. 7 In inside-out patches in which the glibenclamide-sensitive K + channel had run down after exposure to levcromakalim, both uridine 5'-diphosphate (UDP) and MgATP were capable of reactivating the channel. Further application of Mg 2+ to the UDP-reactivated K + channels enhanced the channel activity reversibly. 8 In inside-out patches UDP was capable of activating the glibenclamide-sensitive K + channel without levcromakalim, providing that there was free Mg 2+ present (either UDP in 5 mM EGTA or UDP in 5 mM EDTA with Mg 2+ ). Additional application of levcromakalim caused a further reversible activation of channel opening. 9 In the presence of levcromakalim, application of adenosine 5'-triphosphate (ATP) to the inner surface of the membrane patch inhibited UDP-reactivated channel opening in a concentration-dependent manner. 10 Addition of an untreated cytosolic extract of pig proximal urethra reactivated the glibenclamidesensitive K + channel in the presence of 100 mM levcromakalim in inside-out patches. 11 These results demonstrate the presence in the pig proximal urethra of a glibenclamide-sensitive K + channel that is blocked by intracellular ATP and can be activated by levcromaka...

Regenerative potentials were initiated by depolarizing short segments of single bundles of circular muscle isolated from the gastric antrum of guinea‐pigs. When changes in [Ca2+]i and membrane potential were recorded simultaneously, regenerative potentials were found to be associated with an increase in [Ca2+]i, with the increase starting after a minimum latency of about 1 s. Although the increase in [Ca2+]i was reduced by nifedipine, the amplitudes of the regenerative responses were little changed. Regenerative responses and associated changes in [Ca2+]i were abolished by loading the preparations with the Ca2+ chelator MAPTA‐AM. Regenerative potentials were abolished by 2‐aminoethoxydiphenyl borate (2APB), an inhibitor of IP3 induced Ca2+ release, by N‐ethylamaleimide (NEM), an alkylating agent which blocks activation of G‐proteins and were reduced in amplitude by two agents which block chloride (Cl−)‐selective channels in many tissues. The observations suggest that membrane depolarization triggers IP3 formation. This causes Ca2+ release from intracellular stores which activates Ca2+‐dependent Cl− channels.

These results suggest that in the adult pancreas, TGF-β signalling through SMAD2 is crucial for not only the determination of beta cell mass but also the maintenance of defining features of mature pancreatic beta cells, and that this involves modulation of KATP channel activity.