(A) Schematic sketch of the mouse seminiferous tubule (ST) highlighting 12 stages (I – XII) of the spermatogenic cycle (Hess and De Franca, 2008; Russell, 1990), which are arranged in consecutive order along the length of the tubule. A single layer of flat testicular peritubular cells (TPC; green) lines the tubular wall. Sertoli cells (SC) span the tubule from the basal lamina to the lumen. Developing germ cells (GC) are distributed between Sertoli cells. Spermatogonia are located near the basal membrane. Prophase spermatocytes move across the blood-testis barrier (BTB) to the adluminal compartment where they complete meiosis. The resulting haploid spherical cells (round spermatids) differentiate into elongated spermatids and, eventually, into highly condensed and compartmentalized spermatozoa (spermiogenesis). These mature, yet still immotile germ cells are then released into the lumen (spermiation). E, epididymis; TA, tunica albuginea; inspired by Hess and De Franca, 2008. (B) Immunostaining against α-smooth muscle actin (α-SMA, green) marks TPCs in vitro (Tung and Fritz, 1989). Cell count is determined by nuclear staining (DAPI, blue). Cultures retain high TPC purity for at least two passages (92 ± 2%, n = 1102 (#1); 86 ± 3%, n = 542 (#2)). Dashed line delimits one of the few α-SMA-negative cells. (C) RT-PCR profiling of purinoceptor isoforms in TPC cultures reveals P2rx2, P2rx4, P2rx7, P2ry2, and P2ry6 transcripts. Dashed gray vertical lines indicate cuts in a given gel. (D–H) ATP exposure triggers TPC transmembrane currents. (DI) Phase-contrast micrograph depicting a TPC (dashed line) targeted by a patch pipette (pp). (DII-DIV) Original whole-cell recordings illustrate representative currents in response to ATP stimulation (100 µM (DII) vs. 1000 µM (DIII) and 10 s (DIII) vs. 30 s (DIV), respectively). Vhold = −80 mV. (E, F) Quantification (bar charts; mean ± SEM; n as indicated) reveals saturation of peak current density at ≤100 µM ATP (E) and modest desensitization at a concentration-dependent rate (F). (G) Whole-cell voltage-clamp recordings show ATP-induced currents (100 µM; 10 s) that are potentiated by ivermectin (3 µM) and partially inhibited by suramin (100 µM), respectively (≥60 s preincubation). Vhold = −80 mV. (H) Quantification (bar charts; mean ± SEM; data normalized to initial control response) demonstrates dichotomy in drug sensitivity. Treatment was categorized as effective (eff) if current amplitudes deviate by ± SD from average control recordings (85 ± 24%, 2nd ATP stimulation). Note that each drug proved ineffective (no eff) in some cells. Gray circles depict data from individual cells. (I–L) ATP-dependent Ca2+ mobilization in cultured TPCs. Ca2+ transients in response to repetitive stimulation (10 µM, 10 s) are monitored by ratiometric (fura-2) fluorescence imaging. (I) Phase contrast (I) and merged fluorescence (f380; II) images of a TPC in vitro. Bottom pseudocolor frames (rainbow 256 color map) illustrate relative cytosolic Ca2+ concentration ([Ca2+]c) before (III) and during (IV) ATP stimulation. (J, K) Representative original traces from time-lapse fluorescence ratio (f340/f380) recordings depict repetitive [Ca2+]c elevations upon ATP exposure under control conditions ((J) blue traces correspond to the TPC in (I)) and during reduced extracellular Ca2+ concentration ((K) [Ca2+]e = 100 nM; 60 s preincubation). (L) Bar chart depicting Ca2+ signal amplitudes (mean ± SEM; n as indicated) – normalized to the initial ATP response – under control conditions (gray) vs. low [Ca2+]e (green). Asterisks denote statistically significant differences (*1p=0.001; *2p=0.002; *3p=5.5e−5; *4p=0.0006; *5p=0.02; *6p=0.02; Student t-test (F, L), one-way ANOVA (H)). (M) Representative whole-cell voltage-clamp recordings (Vhold = −80 mV) of ATP-induced inward currents in cultured mouse TPCs. Two components – a fast relatively small current (arrow head) and a delayed lasting current (asterisk) – are triggered repeatedly by successive ATP exposure (100 μM; 90 s inter-stimulus interval). Notably, we never observed a delayed slow current without a fast response. (N) Bar chart quantifying peak densities (mean ± SEM, circles show individual values) of the fast (Ifast; black) and the delayed (Islow; gray) ATP-induced current components (1st stimulation: Ifast 0.37 ± 0.2 pA/pF; Islow 1.58 ± 0.5 pA/pF; 2nd stimulation Ifast 0.27 ± 0.1 pA/pF; Islow 1.37 ± 0.4 pA/pF). (O) Bar graph illustrating the frequency of Ifast (black) and Islow (gray) occurrence upon ATP (100 µM) stimulation in absence (w/o) and presence (w) of GTP (500 μM) in the pipette solution, respectively. Asterisks denote statistically significant differences (*7p=0.008, *8p=0.0003; Fisher‘s exact test); n as indicated in bars. (P) Representative whole-cell voltage-clamp recordings (Vhold = −80 mV) of inward currents induced by ATP (100 μM) and UTP (100 μM), respectively. Whenever ATP triggers both Ifast and Islow (left), Islow is also induced by UTP (right). UTP-dependent currents develop significantly slower than ATP-evoked Ifast (inset; *9p=0.03; paired t-test).