Characterisation of the ionic currents in freshly isolated rat ureter smooth muscle cells: evidence for species-dependent currents

Abstract

To better understand excitability, and hence contraction, the ionic currents underlying the action potential were identified and characterised in enzymatically isolated smooth muscle cells of the rat ureter. Using the whole-cell patch-clamp, under voltage-clamp conditions with K(+) in the pipette, three types of responses occurred to depolarisation: (1) sustained outward current and spontaneous transient outward currents (STOCs); (2) inward current; and (3) fast outward current. Investigation using different voltage protocols and pharmacological blockers and agonists revealed the presence of three outward and two inward currents. The outward currents were: (1) a sustained BK current, sensitive to low concentrations of tetraethylammonium (TEA) and featuring bursts of STOCs superimposed on it; (2) a fast, transient, A-type K current sensitive to 4-aminopyridine; and (3) a TEA and Ca(2+)-insensitive, late K(+) rectifier current. The inward currents were: (1) a fast L-type Ca(2+) channel current sensitive to nifedipine, Cd(2+) and potentiated by Ba(2+); and (2) a Ca(2+)-sensitive Cl(-) channel, which was inhibited by niflumic acid and Ba(2+), and produced a large tail current upon repolarisation at the end of the voltage step. The I- V relationships and peak amplitudes of all the currents are described. The finding of a K(+) rectifier and Ca(2+)-activated Cl(-) channel distinguish the rat ureteric cells from those of the guinea-pig. Thus, as well as the previously established difference in sarcoplasmic reticulum Ca(2+)-release mechanisms, there is also a species difference in ion channel expression in this tissue. We relate these currents to their possible contribution to the characteristically extremely long lasting action potential in the rat ureter.

How to Properly Measure a Current-Voltage Relation?—Interpolation vs. Ramp Methods Applied to Studies of GABAA Receptors

“…Analysis of the I-V relations obtained from the current amplitudes at steady voltages, and often after different times after a voltage step or start of agonist application that activate particular channel types, has been frequently used since the introduction of the voltage clamp technique (Hodgkin et al, 1952 ) to understand the functional properties of various types of ion channels, including neuronal voltage-gated (for examples, see Frankenhaeuser, 1962 ; Johansson and Århem, 1992 ; Williams et al, 1997 ) and ligand-gated channels (Trussell et al, 1988 ; Mittman et al, 1990 ; Zhang and Trussell, 1994 ; Fu et al, 1997 ; Kaila et al, 1997 ; Karlsson et al, 1997a ; Kumar and Huguenard, 2001 ; Bianchi and Macdonald, 2002 ; Christophe et al, 2002 ; Cordero-Erausquin et al, 2005 ), non-neuronal voltage-gated (Miyazaki et al, 1975 ; Yatani et al, 1987 ; Smith et al, 2002 ) and ligand-gated (Magleby and Stevens, 1972 ; Ifune and Steinbach, 1992 ; Sims, 1992 ) channels as well as other channel types (Zhang et al, 1998 ). As an alternative to recording the current at a steady voltage, I-V relations have also been obtained by using voltage ramps, where the controlled voltage changes gradually at a steady rate and simultaneously the current is continuously recorded (for examples concerning voltage-gated channels, see Fishman, 1970 ; Spindler et al, 1999 ; Del Negro et al, 2002 ; Yamada-Hanff and Bean, 2013 ; for ligand-gated channels see Bolton, 1975 ; Adams and Sakmann, 1978 ; Sims, 1992 ; Karlsson et al, 2011 ; for other channel types, see Zhang et al, 1998 ).…”

The effect of trichostatin-A and tumor necrosis factor on expression of splice variants of the MaxiK and L-type channels in human myometrium

“…This quiescent state is important because it facilitates normal growth and development of the baby. The quiescent state is characterized by slow wave potentials where the membrane potential cycles between depolarization and repolarizations without reaching the threshold level to produce an action potential (Parkington et al, 1999; Smith et al, 2002; Aguilar and Mitchell, 2010). At term, this quiescent state ends and these slow wave potentials become frequent synchronized action potentials during which the membrane potential rapidly rises and falls (Wilde and Marshall, 1988)—i.e., the myometrium undergoes the rhythmic contractions seen in labor to ensure the baby is delivered (Khan et al, 1993b, 1997, 1998a,b; Tribe et al, 2000; Shmygol et al, 2007).…”