Pacemaker function and neural responsiveness of subserosal interstitial cells of Cajal in the mouse colon

Abstract

Key points Rhythmic action potentials and intercellular Ca 2+ waves are generated in smooth muscle cells of colonic longitudinal muscles (LSMC). Longitudinal muscle excitability is tuned by input from subserosal ICC (ICC‐SS), a population of ICC with previously unknown function. ICC‐SS express Ano1 channels and generate spontaneous Ca 2+ transients in a stochastic manner. Release of Ca 2+ and activation of Ano1 channels causes depolarization of ICC‐SS and LSMC, leading to activation of L‐type Ca 2+ channels, action potentials, intercellular Ca 2+ waves and contractions in LSMC. Nitrergic neural inputs regulate the Ca 2+ events in ICC‐SS. Pacemaker activity in longitudinal muscle is an emergent property as a result of integrated processes in ICC‐SS and LSMC. Abstract Much is known about myogenic mechanisms in circular muscle (CM) in the gastrointestinal tract, although less is known about longitudinal muscle (LM). Two Ca 2+ signalling behaviours occur in LM: localized intracellular waves not causing contractions and intercellular waves leading to excitation‐contraction coupling. An Ano1 channel antagonist inhibited intercellular Ca 2+ waves and LM contractions. Ano1 channels are expressed by interstitial cells of Cajal (ICC) but not by smooth muscle cells (SMCs). We investigated Ca 2+ signalling in a novel population of ICC that lies along the subserosal surface of LM (ICC‐SS) in mice expressing GCaMP6f in ICC. ICC‐SS fired stochastic localized Ca 2+ transients. Such events have been linked to activation of Ano1 channels in ICC. Ca 2+ transients in ICC‐SS occurred by release from stores most probably via inositol trisphosphate receptors. This activity relied on influx via store‐operated Ca 2+ entry and Orai channels. No voltage‐dependent mechanism that synchronized Ca 2+ transients in a single cell or between cells was found. Nitrergic agonists inhibited Ca 2+ transients in ICC‐SS, and stimulation of intrinsic nerves activated nitrergic responses in ICC‐SS. Cessation of stimulation resulted in significant enhancement of Ca 2+ transients compared to the pre‐stimulus activity. No evidence of innervation by excitatory, cholinergic motor neurons was found. Our data suggest that ICC‐SS contribute to regulation of LM motor activity. Spontaneous Ca 2+ transients activate Ano1 channels in ICC‐SS. Resulting depolarization conducts to SMCs, depolarizing membrane potential, activating L‐type Ca 2+ channels and initiating contraction. Rhythmic electrical and mechanical behaviours of LM are an emergent property of SMCs and ICC‐SS.