DUAL CONTROL OF SEMINIFEROUS TUBULE CONTRACTILITY MEDIATED BY ET(A) AND ET(B) ENDOTHELIN RECEPTOR SUBTYPES

Testicular myoid cells surrounding the seminiferous tubule are contrac tile cells responsible for peritubular contractility and for the propu lsion of tubular fluid and spermatozoa. We have investigated the contr actile response of rat myoid cells to endothelins (ETs) in cell and or gan culture and analyzed the cell signaling involved. ET-2, ET-3, and IRL 1620, a highly selective agonist of ET(B) receptor, elicit [Ca2+]( i) increases, though with dissimilar potencies and kinetics. Competiti on binding assays using [I-125]ET-1, [I-125]ET-3 and [I-125]IRL 1620 s how 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 [Ca2+](i) . Although stimulation of the ET(B) receptor with IRL 1620 fails to pr oduce a significant effect on inositol phosphate (IF) production, it i nduces mobilization of a thapsigargin-sensitive intracellular Ca2+ poo l in the absence of any measurable IP production. We also studied the effect of U-73122 [1-(6-[17-beta-3-methoxyestra-1,3,5 (10)-trien-17-yl ] amino/hexyl)-H-1-pirrole-2,5-dione] and its inactive analog, U-73343 , on Ca2+ mobilization and IP production after selective stimulation o f ET receptors. U-73122 (1 mu M) completely inhibited the effect of ET (A)-mediated ET-1 stimulation of IP production, whereas U-73343 was in active. However, in the presence of U-73122, the selective stimulation of ET(B) receptors induced the mobilization of a thapsigargin-sensiti ve and inositol phosphate-independent intracellular Ca2+ pool. The ET( B) receptor-dependent mobilization of [Ca2+](i) resulted mainly from C a2+ release from intracellular Ca2+ stores. This paper illustrates con traction of myoid cells in the seminiferous tubule in response to sele ctive activation of either ET receptor. Scanning electron microscopy o f the peritubular tissue demonstrates that the contractile response to ET was inhibited by a combination of BQ-123 and BQ-788, but not by ei ther antagonist alone. Moreover, the observation that selective stimul ation of ET(B) receptor with IRL 1620 also resulted in cell contractio n strongly suggests that stimulation of either ET(A) or ET(B) receptor s alone may be sufficient to elicit seminiferous tubule contractility, Two types of receptors account for the actions of endothelin on contr actile activity of seminiferous tubule: 1) an ET(A) receptor that is p ositively coupled to phospholipase C (PLC) and Ca2+ mobilization; and 2) an ET(B) receptor that induces title mobilization of a thapsigargin -sensitive intracellular Ca2+ pool in a manner independent of the form ation of inositol phosphates. ET may play a complex role in regulating the flux of spermatozoa along the seminiferous tubule through its con tractile effect on peritubular myoid cells.-Tripiciano, A., Palombi, F ., Ziparo, E., Filippini, A. Dual control of seminiferous tubule contr actility mediated by ET(A) and ET(B) endothelin receptor subtypes.