Dual control of seminiferous tubule contractility mediated by ET A and ET B endothelin receptor subtypes

Testicular myoid cells surrounding the seminiferous tubule are contractile cells responsible for peritubular contractility and for the propulsion of tubular fluid and spermatozoa. We have investigated the contractile response of rat myoid cells to endothelins (ETs) in cell and organ culture and analyzed the cell signaling involved. ET‐2, ET‐3, and IRL 1620, a highly selective agonist of ET B receptor, elicit [Ca 2+ ]i increases, though with dissimilar potencies and kinetics. Competition binding assays using [ 125 I]ET‐1, [ 125 I]ET‐3 and [ 125 I]IRL 1620 show that myoid cells express both ET A and ET B receptors with high affinity for ET‐1 and ET‐1/ ET‐3, respectively. All endothelin isoforms activate phosphoinositide (PI) turnover, but only stimulation of the ET A receptor mediates both PI turnover and mobilization of [Ca 2+ ] i . Although stimulation of the ET B receptor with IRL 1620 fails to produce a significant effect on inositol phosphate (IP) production, it induces mobilization of a thapsigargin‐sensitive intracellular Ca 2+ pool in the absence of any measurable IP production. We also studied the effect of U‐73122 [l‐(6‐[17‐β‐3‐methoxyestra‐1,3,5 (10)‐trien‐17‐yl] amino/hexyl)‐ 1 H‐pirrole‐2,5‐dione] and its inactive analog, U‐73343, on Ca 2+ mobilization and IP production after selective stimulation of ET receptors. U‐73122 (1 μM) completely inhibited the effect of ET A ‐mediated ET‐1 stimulation of IP production, whereas U‐73343 was inactive. However, in the presence of U‐73122, the selective stimulation of ET B receptors induced the mobilization of a thapsigargin‐sensitive and inositol phosphate‐independent intracellular Ca 2+ pool. The ET B receptor‐dependent mobilization of [Ca 2+ ] i resulted mainly from Ca 2+ release from intracellular Ca 2+ stores. This paper illustrates contraction of myoid cells in the seminiferous tubule in response to selective activation of either ET receptor. Scanning electron microscopy of the peritubular tissue demonstrates that the contractile response to ET was inhibited by a combination of BQ‐123 and BQ‐788, but not by either antagonist alone. Moreover, the observation that selective stimulation of ET B receptor with IRL 1620 abo resulted in cell contraction strongly suggests that stimulation of either ET A or ET B receptors alone may be sufficient to elicit seminiferous tubule contractility. Two types of receptors account for the actions of endothelin on contractile activity of seminiferous tubule: 1 ) an ET A receptor that is positively coupled to phospho‐lipase C (PLC) and Ca 2+ mobilization; and 2) an ET B receptor that induces the mobilization of a thapsi‐gargin‐sensitive intracellular Ca 2+ pool in a manner independent of the formation of inositol phosphates. ET may play a complex role in regulating the flux of spermatozoa along the seminiferous tubule through its contractile effect on peritubular myoid cells.—Tripiciano, A., Palombi, F., Ziparo, E., Filippini, A. Dual control of seminiferous tubule contractility mediated by ET A and ET B endothelin receptor subtypes. FASEB J. 11, 276‐286 (1997)