Benign prostatic hyperplasia (BPH) is a stromal disease that subsequently affects epithelial growth. Growth factors activate downstream intracellular pathways, but the linkage to cellular proliferation is unclear. In many cells growth factors raise intracellular Ca²⁺ (de Laat et al. 1999) and Ca²⁺ channel activity correlates with cell proliferation. T-type channels are prominent during developmental cell growth, and activity is modulated by growth hormone or conditions leading to cellular hypertrophy and proliferation (Guo et al. 1998), but the channels are less evident in differentiated cells. Ca²⁺ channels have not been characterised in human prostate smooth muscle, and the aim was to carry out an analysis prior to an investigation of the cellular functions of growth factors.

Prostate samples were obtained, with Ethical Committee approval, from patients undergoing either transurethral resection of the prostate (TURP) or radical prostatectomy. They were stored in Ca²⁺-free Hepes-Tyrode solution. Isolated cells were prepared by collagenase-based digestion and, in general, this was easier with prostatectomy samples as retrieved tissue was less damaged. Experiments were carried out in HCO₃^–/CO₂-buffered Tyrode solution at 37 °C. Whole-cell recordings used patch-electrodes, and whole-cell capacitance was measured on membrane rupture to scale membrane currents to unit capacitance. Cs⁺-based electrode solutions, to block outward currents, were used to record inward currents, and K⁺-based solutions to record resting potentials. Data are means ± S.D., and differences between data sets (P < 0.05) were tested using Student’s unpaired t test.

The resting potential was -63 ± 11 mV (n = 8) and action potentials were elicited with Cs⁺-filled electrodes; maximum upstroke velocity was about 0.8 V s^-1. Inward current was dependent on superfusate Ca²⁺ with peak current at 0 mV. Current density (holding potential, V_h, (-100 mV) was significantly less in cells from TURP chips (72 ± 6 pF) compared to prostatectomy (71 ± 5 pF) samples (1.9 ± 0.27 vs. 3.3 ± 0.40 pA pF^-1, n = 14 and 24, respectively), otherwise there were no differences between the two groups. The current could be divided into two components (L-type and T-type) as determined by the dependence on V_h -100 or -40 mV) or their sensitivity to 30 µM verapamil and 20 µM NiCl₂. Separate experiments showed that these were maximal concentrations of blockers. Half-maximum activation was -7.2 ± 1.0 mV for L-type and -36.1 ± 2.0 mV for T-type current (n = 15). The L-type current was the larger component. At their respective maximum voltages L-type current density was 1.56 ± 0.47 pA pF^-1 (+10 mV) and T-type current was 0.83 ± 0.16 pA pF^-1 (-20 mV).

This is the first study to characterise systematically inward current in human prostate smooth muscle. Mean peak net inward current is about 60 % of that in detrusor (Sui et al. 2001) and has two components that have properties of L-type and T-type channels. Of interest is the Ni²⁺ dependency of the T-type component as there are several isoforms of the α-subunit. The α_1G subtype has a low affinity for Ni²⁺ compared to the α_1H subtype (Lee et al. 1999). In detrusor the low affinity subtype is present (Sui et al. 2001) but in prostate the low [NiCl₂] (20 µM) used for block suggests the α_1H subtype. Thus any manipulation of prostate channel activity could have selective effects over detrusor.

We thank the Wellcome Trust for financial assistance and the urological surgeons at the Middlesex and St George’s Hospitals for their help