Heterogeneity of the Ca2+ sensitivity of secretion in a pituitary gonadotrope cell line and its modulation by protein kinase C and Ca2+

✍ Scribed by Hui-Sheng Liu; Zhi-Tao Hu; Ke-Ming Zhou; Ya-Ming Jiu; Hua Yang; Zheng-Xing Wu; Tao Xu

• Publisher: John Wiley and Sons
• Year: 2006
• Tongue: English
• Weight: 384 KB
• Volume: 207
• Category: Article
• ISSN: 0021-9541
• DOI: 10.1002/jcp.20598

✦ Synopsis

Abstract

Modulation of the Ca^2+^ sensitivity and cooperativity of secretion is an important means of regulating neurotransmission and hormone secretion. Employing high‐time resolution measurement of membrane capacitance (Cm) stimulated by step‐like or ramp [Ca^2+^]~i~ elevation, we have identified the co‐existence of both a high and low Ca^2+^‐sensitive exocytosis in an immortal pituitary gonadotrope cell line, LβT2. Ramp [Ca^2+^]~i~ generated by slow uncaging elicited a biphasic C~m~ response. The first phase of response, which represents a highly Ca^2+^‐sensitive pool (HCSP) of vesicles, began to secrete at low [Ca^2+^]~i~ concentration (<1 µM) with low Ca^2+^ cooperativity. In contrast, the second phase, which represents a lowly Ca^2+^‐sensitive pool (LCSP) of vesicles, only exocytozed at higher [Ca^2+^]~i~ (>5 µM) and displayed a steep Ca^2+^ cooperativity. The co‐existence of vesicle populations with different Ca^2+^ sensitivities was further confirmed by flash photolysis stimuli. The size of the HCSP was ∼30 fF under resting conditions, but was dramatically increased (∼ threefold) by application of phorbol‐12‐myristate‐13‐acetate (PMA, an activator of protein kinase C). Forskolin (an activator of protein kinase A), however, exerted no significant effect on the size of both HCSP and LCSP. GnRH (gonadotropin releasing hormone) augmented the size of both pools to a larger extent (5‐ and 1.7‐fold increase for HCSP and LCSP, respectively). The heterogeneity of Ca^2+^ sensitivity from different pools of vesicles and its differential modulation by intracellular signals suggests that LβT2 cells are an ideal model to further unravel the mechanism underlying the modulation of Ca^2+^‐sensing machineries for exocytosis. J. Cell. Physiol. © 2006 Wiley‐Liss, Inc.