Keeping Pace with Interstitial Cells of Cajal: Modelling Gastrointestinal Electrophysiology

Abstract

Gastrointestinal (GI) motility is coordinated by several cooperating mechanisms, including myogenic, neural and hormonal control systems. This thesis focuses on one of these mechanisms: an intrinsic bioelectrical activity called slow waves, which originates in pacemaker cells called interstitial cells of Cajal (ICC) located within the smooth muscle layers of the GI tract. The mechanism by which ICC generate slow waves is a matter of ongoing research, although rhythmic calcium (Ca2+) oscillations are known to underlie slow wave activity, and both voltage- and Ca2+-dependent ion channels are involved in the pacemaker mechanism. In this thesis, mathematical models of pacemaker activity were developed to investigate the mechanisms by which slow waves are generated and regulated in ICC. The literature was reviewed to determine the ion channels and Ca2+ dynamics likely to contribute to ICC pacemaker activity, particularly the identity of the pacemaker channel that initiates the slow wave and the channels that contribute to the characteristic plateau phase of the slow wave. A pacemaker hypothesis was proposed in which the pacemaker channel is a Ca2+-activated chloride (Cl ) channel called anoctamin 1 (Ano1), which is activated by a localised increase in intracellular Ca2+ concentration ([Ca2+]i). Cyclical release of Ca2+ from intracellular stores is believed to initiate pacemaker activity, so Ano1 was proposed to be activated by Ca2+ in ux through store-operated Ca2+ (SOC) channels. A novel mathematical model of Ano1 current was constructed. The Ano1 model reproduced experimentally observed behaviour, including the steady-state voltage- and Ca2+-dependent activation pro le, slow activation at low [Ca2+]i, rapid activation at high [Ca2+]i, and slow deactivation when [Ca2+]i is reduced. The Ano1 model was then incorporated into a new compartmental model of small intestinal ICC pacemaker activity based on the proposed pacemaker hypothesis. A series of simulations were carried out using the ICC model to investigate current controversies about the reversal potential of the Ano1 Cl current in ICC, and to predict the characteristics of the other ion channels that are necessary to generate slow waves. The model showed that Ano1 is a likely pacemaker channel when coupled to a SOC channel, but predicted that ICC in Ano1 knockout mice may still generate small cyclical depolarisations despite the absence of the pacemaker channel. The results suggested that voltage- or Ca2+-activated non-selective channels or sodium (Na+) channels may contribute to the slow wave plateau phase, whereas voltage-dependent Ca2+ current is likely to be negligible during the plateau. The Cl equilibrium potential was shown to be an important modulator of slow wave morphology, highlighting the need for a better understanding of Cl dynamics in ICC in order to clarify how Ano1 and other Cl currents contribute to the slow wave plateau and repolarisation. The spontaneous pacemaker activity of ICC is also regulated by mechanical inputs. An original model of a mechanosensitive Na+ channel found in human small intestine ICC was developed and incorporated into a previously published small intestine ICC model. Simulation results showed that mechanosensitive changes in the Na+ current caused up to 5% depolarisation of resting membrane potential, 11% increase in slow wave upstroke rate, 5% increase in slow wave duration, and 1% increase in frequency. These results were comparable to the experimentally observed e ects of stretching smooth muscle tissue, indicating that Na+ channel mechanosensitivity can explain the e ects of stretch on slow waves. In summary, this thesis presents: a new model of Ano1 current; the rst ICC model to implement Ano1 as a pacemaker channel and to include store-operated Ca2+ entry as a component of the pacemaker cycle; and the rst model of slow wave regulation by mechanical stimuli.