Partial agonistic effects of pilocarpine on Ca2+ responses and salivary secretion in the submandibular glands of live animals. Issue 6 (1st June 2015)

• Record Type: Journal Article
• Authors: Nezu, Akihiro; Morita, Takao; Tojyo, Yosuke; Nagai, Takeharu; Tanimura, Akihiko
• Is Part Of: Experimental physiology. Volume 100:Issue 6(2015:Jun.)
• Page Start: 640
• Page End: 651
• Publication Date: 2015-06-01
• Subjects: Physiology, Experimental -- Periodicals; 571.0724
• DOI: 10.1113/EP085110
• Languages: English
• ISSNs: 0958-0670

Abstract

<abstract abstract-type="main"> <title> <x xml:space="preserve">Abstract</x> </title> <sec id="eph1633-sec-0010" sec-type="section"> <title>New Findings</title> <p> <list id="eph1633-list-0001" list-type="bullet"> <list-item> <p> <bold>What is the central question of this study?</bold> </p> <p>Pilocarpine stimulates salivary secretion via muscarinic ACh receptors (mAChRs), although the Ca<sup>2+</sup>‐mobilizing effect of pilocarpine in salivary gland cells is extremely small. Therefore, we examined the effect of pilocarpine on Ca<sup>2+</sup> responses in submandibular gland cells and on secretion <italic>in vitro</italic> and <italic>in vivo</italic>.</p> </list-item> <list-item> <p> <bold>What is the main finding and its importance?</bold> </p> <p>Pilocarpine induces small Ca<sup>2+</sup> responses and reduces the effects of other mAChR agonists on Ca<sup>2+</sup> responses via its partial agonistic effects. These effects of pilocarpine on Ca<sup>2+</sup> responses in the submandibular gland were further established <italic>in vivo</italic> with a novel Ca<sup>2+</sup> imaging system and a genetically encoded Ca<sup>2+</sup> indicator.</p> </list-item> </list> </p> </sec> <sec id="eph1633-sec-0020" sec-type="section"> <p>Pilocarpine stimulates salivary secretion via muscarinic ACh receptors (mAChRs), although the effect of pilocarpine on Ca<sup>2+</sup> responses in dispersed salivary gland cells is extremely small. Here, we demonstrate the effect of pilocarpine on Ca<sup>2+</sup> responses and salivary secretion in the rat submandibular gland (SMG). In fura‐2‐loaded SMG cells, the maximal effect of pilocarpine on [Ca<sup>2+</sup>]<sub>i</sub> elevation was 16% of that of carbachol, and pilocarpine attenuated carbachol‐ and bethanechol (Bet)‐induced [Ca<sup>2+</sup>]<sub>i</sub> increases, indicating that pilocarpine acts as a partial agonist for mAChR‐mediated Ca<sup>2+</sup> responses. The partial agonistic effect of pilocarpine on Ca<sup>2+</sup> dynamics in the SMG was also confirmed in live animals using the genetically encoded Ca<sup>2+</sup> indicator, YC‐Nano50. Administration of pilocarpine (3 mg kg<sup>−1</sup>, <sc>i.p.</sc>) elicited a small increase in [Ca<sup>2+</sup>]<sub>i</sub> in the SMG. Quantitative analyses demonstrated that resting [Ca<sup>2+</sup>]<sub>i</sub> was ∼37 n<sc>m</sc>, which was increased by pilocarpine (3 mg kg<sup>−1</sup>) and Bet (10 mg kg<sup>−1</sup>) to 44 and 69 n<sc>m</sc>, respectively. The inhibitory effects of pilocarpine on Bet‐induced Ca<sup>2+</sup> responses were also elucidated <italic>in vivo</italic>. We further examined real‐time changes in pilocarpine‐induced SMG salivary secretion and showed that pilocarpine induced an extremely weak secretory response and reduced Bet‐induced secretion. Unlike Ca<sup>2+</sup> responses, pilocarpine failed to reduce the effect of Bet on SMG blood flow. Our results demonstrate that pilocarpine acts as a partial agonist of mAChRs to induce weak salivary secretion that is correlated with small increases in [Ca<sup>2+</sup>]<sub>i</sub>. Furthermore, pilocarpine exhibits an antagonistic effect on mAChR‐induced Ca<sup>2+</sup> responses and salivary secretion.</p> </sec> </abstract>