T‐ and L‐type Ca²⁺ Currents in Freshly Dispersed Smooth Muscle Cells from the Human Proximal Urethra

Abstract

The urethra generates sufficient tone to prevent leakage from the bladder and thus plays an important role in maintaining urinary continence. Despite this central role, relatively little is known about the mechanisms that underlie the generation and modulation of urethral tone, although it can be influenced by a number of factors including blood flow through the lamina propria . There is little doubt that a myogenic mechanism also contributes significantly to urethral tone, since it is unaffected in vitro by nerve blockade in a variety of species including pigs (Bridgewater et al. 1993), sheep (Thornbury et al. 1992, rats (McKeag et al. 2001) and humans .A number of studies have demonstrated that urethral myogenic tone is critically dependent on the influx of Ca 2+ across the cell membrane, since removal of external Ca 2+ or inhibition of L-type Ca 2+ channels reduces tone significantly in rats, humans and pigs in vitro (Bridgewater et al. 1993;Brading, 1999;Shafei et al. 2003). Shafei et al. (2003) have demonstrated that application of nifedipine or Ni 2+ significantly reduces tone in an isolated rat whole urethra preparation, suggesting that Ca 2+ influx through both T and L channels contributes to urethral tone. Recent studies by Bradley et al. (2003) have characterised the Ca 2 currents in isolated rabbit urethral myocytes and demonstrated the existence of currents with biophysical and pharmacological properties typical of L-and T-type Ca 2+ currents in arterial (Benham et al. 1987), venous (Yatani et al. 1987) and bladder myocytes (Sui et al. 2001).To date, no study has examined successfully the electrophysiology of human urethral myocytes, presumably because of the poor availability of suitable tissue and the difficulty of obtaining viable cells from small biopsy samples. In this study we provide the first electrophysiological data from freshly dispersed human myocytes obtained from adults undergoing treatment for bladder or prostate cancer. Our results suggest that human urethral myocytes possess Ca 2+ currents with electrophysiological and pharmacological properties typical of T and L channels (for reviews see Kotlikoff et al. 1999;Perez-Reyes, 2003 The purpose of the present study was to characterise Ca 2+ currents in smooth muscle cells isolated from biopsy samples taken from the proximal urethra of patients undergoing surgery for bladder or prostate cancer. Cells were studied at 37°C using the amphotericin B perforated-patch configuration of the patch-clamp technique. Currents were recorded using Cs

Inhibitory actions of mibefradil on steroidogenesis in mouse Leydig cells: involvement of Ca2+ entry via the T-type Ca2+ channel

“…As previous studies have shown, I CaT was recorded at depolarization (−30 mV) in complete isolation from the L-type Ca 2+ current [30]. Not surprisingly, I CaT was highly sensitive to mibefradil.…”

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

“…This is given weight because the resting membrane potential of these smooth muscle is about -50 to -60 mV from which spontaneous fluctuations of potential are often superimposed ( figure 1B); similar to the window current of Ca 2+ influx via this channel, due to the significant overlap of steady-state activation and inactivation curves -see figure 1C [7][8][9][10][11]. Furthermore, the action potential threshold voltage becomes more negative, as the membrane potential is hyperpolarised [7].…”

“…T-type Ca 2+ current has been recorded in isolated smooth muscle cells from the detrusor layer of human and animal bladders [7,8], from human and animal urethra [9,10] and from human prostate samples [11].…”

“…In each tissue T-type current contributes 20-30% of the total Ca 2+ current and represents a peak inward current of about 1 pA.pF -1 in detrusor cells. There are however, qualitative differences in current characteristics from the different tissues, for example Ni 2+ sensitivity is greater with current from urethral compared to detrusor myocytes [7,9]. This may reflect different α1 subunits that comprise the channel pore, as Ni 2+ has variable potency on channels composed of these various subunits [1].…”

Electrical slow waves in the mouse oviduct are dependent on extracellular and intracellular calcium sources

“…Previous studies have shown that slow waves of the proximal colon (24,54), stomach (15,29), and small intestine (9,14) ] i in smooth muscles can occur either by influx through ion channels in the plasma membrane or by release from intracellular stores (41). Cav1.2 channels, commonly expressed in smooth muscle cells, are activated by high voltages with maintained "window currents" in the range Ϫ50 to Ϫ10 mV (7,16,23). Although L-type calcium channels play a prominent role in E-C coupling in smooth muscle cells, they are not involved in the generation of slow waves in other visceral smooth muscles (1,21,24,39,46,55).…”