Identification of the major membrane currents in freshly dispersed single smooth muscle cells of guinea‐pig ureter.

Abstract

SUMMARY1. The passive and active electrical properties of freshly dispersed single cells of the guinea-pig ureter were investigated using standard patch-clamp techniques.2. Action potentials, having a rapid rising phase and a prolonged plateau, were recorded on passing depolarizing currents through the patch pipette when 'nearnormal' physiological gradients were established across the cell membrane (5-9 mM-K+, 1-5 mM-Ca2" in the bath; 126 mM-K' in the pipette).3. Under voltage clamp, depolarization to potentials positive of -50 mV (from a holding potential of -70 or -80 mV) triggered a net inward current which reached a peak in 5-10 ms and then slowly inactivated.4. The averaged membrane current to depolarization to potentials between 30 and 0 mV showed two distinct patterns after the peak of the inward current; the membrane current either moved slowly outward over 400 ms or there was one or more transient outward currents superimposed on the slowly decreasing inward current. Both outward currents were blocked when 5 mM-tetraethylammonium (TEA) was added to the bathing solution, resulting in an increased inward current at all potentials.5. Replacing the extracellular Ca2+ with C02+ (15-5 mM) blocked the inward current and the outward currents to reveal another transient outward current (voltage activated) which activated rapidly to reach a peak within 5 ms and which inactivated exponentially with a time constant of 10 ms. This voltage-activated outward current was inactivated if the membrane was held at -50 mV, but could be reactivated by short hyperpolarizing pre-pulses. The amplitude of this transient current in response to a fixed depolarization (to 0 mV) was half-maximum when the hyperpolarizing pre-pulse was to -66 mV. The voltage-activated outward current was reduced in amplitude when the extracellular potassium was raised to 4;6 mM or upon exposure to 1 mM-4-aminopyridine (4-AP), but was not affected by 5 4M-TEA.

Outward Currents in Smooth Muscle Cells Isolated from Sheep Mesenteric Lymphatics

“…In most of our experiments 4-AP had little differential effect on peak current at the beginning of the 500 ms test pulse compared with the sustained current at the end of the pulse (Fig. 6A-C), thus providing little evidence of selective blockade of a transient current (Lang, 1989). The delayed rectifier current in lymphatic cells may be mediated by several K¤ channel subtypes, as TEA and 4_AP appeared to block different components of current.…”

“…Voltage-dependent K¤ currents in smooth muscle fall into two broad groups, (1) 'transient outward' currents characterized by rapid activation and inactivation kinetics, steep voltage-dependent inactivation (V½ negative to −60 mV), sensitivity to 4-AP and resistance to TEA, and (2) 'delayed rectifier' currents which inactivate more slowly, inactivate over a more positive voltage range (V½ = −40 mV) and are variably sensitive to blockade by TEA and 4-AP. Examples of the former are found in guinea-pig ureter (Lang, 1989;Imaizumi, Muraki & Watanabe, 1990), rabbit portal vein (Beech & Bolton, 1989a), rat ileum (Smirnov, Zholos & Shuba, 1992) and guinea-pig proximal colon (Vogalis, Lang, Bywater & Taylor, 1993), while examples of the latter are found in rabbit pulmonary artery (Okabe, Kitamura & Kuriyama, 1987), rabbit portal vein (Beech & Bolton, 1989b), and canine proximal colon (Thornbury et al 1992a;Thornbury, Ward & Sanders, 1992b;Carl, 1995). The current described in the present study falls more readily into the second group as it inactivated with a time constant of around 100 ms and had an inactivation V½ of −41 mV.…”

T-type Ca2+ channels in non-vascular smooth muscles

“…However, in this tissue either only an L-type Ca 2+ -current was identified (guinea-pig cells; [18,19]), or in rat cells a second, slower component, attributed to a Ca 2+ -sensitive Cl -channel [20]. The absence of a T-type current in the above experiments was in spite of a sufficiently negative holding potential (-80 to -100 mV).…”

“…Ca 2+ currents have also been characterised in smooth muscle from the ureter [18][19][20]. However, in this tissue either only an L-type Ca 2+ -current was identified (guinea-pig cells; [18,19]), or in rat cells a second, slower component, attributed to a Ca 2+ -sensitive Cl -channel [20].…”

Pacemaker activity in the upper urinary tract

“…In the guinea pig there are oscillations on the plateau of the action potential which appear to depend on the repetitive activation of an inward Ca 2+ current and of an outward Ca 2+ -dependent K + current (Kuriyama and Tomita, 1970;Imaizumi et al, 1989). Repolarization is due to activation of an outward Ca 2+ -dependent K + current (IK(Ca)) which is sensitive to inhibition by tetraethylammonium (TEA) and charbydotoxin and of a voltage dependent Ca 2+ -insensitive outward K + current (ITO) that is TEA insensitive and 4-aminopyridine (4-AP) sensitive (Imaizumi et al, 1989(Imaizumi et al, , 1990Lang, 1989;Sui and Kao, 1997c).…”