Pharmacological characterization of the inwardly‐rectifying current in the smooth muscle cells of the rat bladder

Abstract

1 In freshly-isolated single cells of the rat bladder detrusor, outwardly-rectifying and inwardlyrectifying membrane currents were identified by the whole-cell voltage-clamp technique. 2 The inwardly-rectifying current (IIR) exhibited features of a cation current permeable to both K+ and Na+ but it was unaffected by changes in extracellular Ca2 . It had an activation threshold close to -60 mV and an estimated reversal potential of -29 mV.3 IIR activated slowly with a voltage-sensitive time-constant of 69 ms at -140 mV and 209 ms at -100 mV but it did not exhibit time-dependent inactivation. 4 IIR was unaffected by tetraethylammonium (up to 20 mM) but it was reduced by extracellular Ba2+(1 mM) and by extracellular Cs' (1 mM). 5IIR was reduced by terikalant (100 gM) and markedly inhibited by ciclazindol (100 gM) although at these concentrations, both agents also reduced outward currents. 6 IIR was inhibited by ZD7288 (10-100 iM) in a concentration-dependent manner. At concentrations up to 30 gM, ZD7288 did not reduce the magnitude of outward currents but these were inhibited by 100 gM ZD7288. 7 In strips of bladder detrusor, spontaneous mechanical activity was increased by ZD7288 (0.3-100 gM) and by ciclazindol (0.3-100 gM) but was unaffected by glibenclamide (1-10 gM).8 It is concluded that AIR closely resembles the hyperpolarization-activated current, Ih, previously described in the smooth muscle of rabbit jejunum and in a variety of other cell types. This current may play an important role in modulating detrusor excitability but this could not be confirmed using the inhibitors ZD7288 and ciclazindol. Keywords: Whole-cell voltage-clamp; rat bladder detrusor; smooth muscle; inward-rectifier; Ih; ZD7288; ciclazindol; terikalant; glibenclamide; tetraethylammonium; Ba2"; Cs' Introduction Inwardly-rectifying conductances are absent from most types of smooth muscle (see Bolton & Beech, 1992). However, in the gastro-intestinal tract (rabbit jejunum: Benham et al., 1987), in certain arterioles (submucosal and cerebral: Edwards & Hirst, 1988;Quayle et al., 1993) and in guineapig detrusor (Inoue & Brading, 1990), inwardly-rectifying currents have been described.In rabbit jejunum these show some characteristics of the hyperpolarization-activated cation current (Ih) described in the cells of the sino-atrial node (DiFrancesco et al., 1986), in salamander rod photoreceptors (Wollmuth & Hille, 1992) and in a variety of neurones (Pape, 1996). Ih (Pape, 1996). In contrast, the arteriolar inwardly-rectifying current is K+-selective and is inhibited by external Ba2+ in the range 1-10 ,M (Quayle et al., 1993). In guinea-pig detrusor, neither the purinoceptor-gated nor the stretch-activated inwardly-rectifying currents exhibit selectivity for monovalent over divalent cations (Inoue & Brading, 1990).In preliminary studies, a current (VIR) with inwardly-rectifying properties has been detected in freshly-isolated smooth muscle cells from rat detrusor (Green et al., 1996). The objective of the present investigation was to determin...

Properties of a tonically active, sodium-permeable current in mouse urinary bladder smooth muscle

“…In rat UBSM cells an inward cation current demonstrating a slow time-dependent activation (time constant ϳ200 ms at Ϫ100 mV) on hyperpolarization and block by 1 mM Cs ϩ has been reported (12). Green and colleagues (12) concluded that this cation current was similar to the hyperpolarization-activated cation current (I h ) extensively characterized in cardiac myocytes and neurons (32).…”

“…6 -8) and demonstrates an insensitivity to blockade by 2 mM external Cs ϩ (n ϭ 3). Moreover, Green and colleagues (12) used leak subtraction in their study, eliminating any time-independent background currents, such as the UBSM I cat described here. 9.…”

“…Green and colleagues (12) concluded that this cation current was similar to the hyperpolarization-activated cation current (I h ) extensively characterized in cardiac myocytes and neurons (32). Mouse UBSM I cat , identified here, is significantly different from I h in rat UBSM, because it activates in a time-independent manner with membrane hyperpolarization (Figs.…”

Hyperpolarization-Activated Cation Channels: From Genes to Function

“…In addition, I h was also found in some other tissues such as B cells of pancreatic Langerhans islets (134), enteric (37,179,429), lymphatic smooth muscle (254), uterine smooth muscle (296), smooth muscle cells of the bladder (163) or portal vein (164), testis (341), and the enteric nervous system (427). However, HCN channel transcripts have only been reported in some of these tissues or cell types (134,164,341,427).…”

Ih Blockers Have a Potential of Antiepileptic Effects

“…In spite of many in vitro reports on Ih channels (8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24) as well as in vivo studies on the pacemaking function of Ih (25)(26)(27)(28), any contributions of Ih to epileptic activity are unknown in vivo, except for the one that showed the long-lasting enhancement of the widely expressed intrinsic Ih that converts the potentiated synaptic inhibition to hyperexcitability in the hippocampus of rats that had febrile seizures (29).…”