Effects of iodoacetate on spontaneous electrical activity, slow wave, in the circular muscle of the guinea-pig gastric antrum

Electrophysiological properties of gastric pacemaker potentials

Electrophysiological properties of pacemaker potentials recorded from myenteric interstitial cells of Cajal (ICC-MY) within the guinea-pig gastric antrum are reviewed briefly. Pacemaker potentials consist of two components, a primary component forming a transient depolarization with a rapidly rising initial phase, followed by a secondary component as a plateau with sustained depolarization. The primary component is inhibited by low [Ca2+]o solutions or depolarization of the membrane with high [K+]o solutions. This inhibition could be mimicked by chelating [Ca2+]i with BAPTA-AM, suggesting that this component is produced by activation of voltage-dependent Ca2+ permeable channels. The plateau component is inhibited by low [Cl-]o solution or DIDS, an inhibitor of Ca2+-activated Cl(-)-channels, suggesting that this component is formed by Ca2+-activated Cl(-)-currents. Reduction of Ca2+ release from internal stores by inhibiting the internal Ca-pump with cyclopiazonic acid results in a shortened duration of the plateau component, with no alteration in the rate of rise of the primary component. 2-APB, an inhibitor of the IP3-receptor mediated Ca2+ release from internal stores, abolishes pacemaker potentials, suggesting that the release of Ca2+ from internal IP3-sensitive Ca2+ stores is required for generation of pacemaker potentials. CCCP, a mitochondrial protonophore, depolarizes the membrane and abolishes pacemaker potentials, suggesting that mitochondrial Ca2+ handling functions may be coupled with generation of pacemaker potentials. These results indicate that the two components of pacemaker potentials are generated by different mechanisms; the primary component may be produced by activation of voltage-dependent Ca2+-permeable channels, while the plateau component may be produced by the opening of Ca2+-activated Cl(-)-channels. It is hypothesized that pacemaker potentials are initiated by depolarization of the membrane due to generation of unitary potentials in response to mitochondrial Ca2+ handling. Activation of voltage-dependent Ca2+ influx, IP3-receptor mediated Ca2+ release from the internal stores and Ca2+-activated Cl(-)-channels may be involved as successive steps downstream to the generation of unitary potentials.

Pacemaker frequency is increased by sodium nitroprusside in the guinea pig gastric antrum

In the guinea pig gastric antrum, the effects of sodium nitroprusside (SNP), an NO donor, on pacemaker potentials were investigated in the presence of nifedipine. The pacemaker potentials consisted of primary and plateau components; SNP (> 1 microM) increased the frequency of occurrence of these pacemaker potentials, while inhibiting the plateau component. 1H-[1,2,4]-Oxadiazole [4,3-a] quinoxalin-1-one, an inhibitor of guanylate cyclase, had no effect on the excitatory actions of SNP on the frequency of pacemaker potentials. Other types of NO donor, (+/-)-S-nitroso-N-acetylpenicillamine, 3-morpholino-sydnonimine and 8-bromoguanosine 3'5'-cyclic monophosphate had no excitatory effect on pacemaker activity. Forskolin, an activator of adenylate cyclase, or 4,4'-diisothiocyano-stilbene-2,2'-disulphonic acid, an inhibitor of the Ca(2+)-activated Cl(-) channel, strongly attenuated the generation of pacemaker potentials, and SNP added in the presence of these chemicals restored the generation of pacemaker potentials. The pacemaker potentials evoked by SNP were abolished in low-Ca(2+) solution or by membrane depolarization with high-K(+) solution. The SNP-induced generation of pacemaker potentials was not prevented by cyclopiazonic acid, an inhibitor of internal Ca(2+)-ATPase, but was limited to a transient burst by iodoacetic acid, an inhibitor of glycolysis, carbonyl cyanide m-chlorophenyl-hydrazone, a mitochondrial protonophore, or 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester, an intracellular Ca(2+) chelator. These results suggest that the SNP-induced increase in the frequency of pacemaker potentials is related to the elevated intracellular Ca(2+) concentrations due to release from mitochondria, and these actions may be independent of the activation of guanylate cyclase.

Effects of removal of Na(+) and Cl(-) on spontaneous electrical activity, slow wave, in the circular muscle of the guinea-pig gastric antrum

In the circular muscle of the guinea-pig gastric antrum, a decrease in the external Na(+) to less than 20 mM produced depolarization of the membrane with transient prolongation of the slow wave. This was followed by a high rhythmic activity. The activity was inhibited by reapplication of Na(+) before recovery. The depolarization in Na(+)-deficient solution was prevented and rhythmic activity continued at about 5/min for at least 6 min by simultaneous removal of K(+), Ca(2+), or Cl(-). After exposure to a Na(+)- and Cl(-)-deficient solution for a few minutes, reapplication of the Na(+) in Cl(-)-deficient solution inhibited generation of the slow wave until Cl(-) reapplication. Similar results were obtained when Na(+) and Cl(-) were reapplied in the absence of K(+) after exposure to a Na(+)-, K(+)-free, and Cl(-)-deficient solution, although the inhibition was weaker than Na(+) reapplication in a Cl(-)-deficient solution. In the presence of furosemide or bumetanide, a strong inhibition of activity was produced by the reapplication of Na(+) and Cl(-) after exposure to an Na(+)- and Cl(-)-deficient solution. A hypothesis is presented that intracellular Ca(2+) concentration ([Ca(2+)](i)) is the most important factor determining the generation and frequency of the slow wave and that [Ca(2+)](i) is regulated by the Na(+) concentration gradient across the plasma membrane. The recovery of the Na(+) concentration gradient by Na(+) reapplication after removal of Na(+) and Cl(-) is mainly controlled by a Na(+)-K(+)-Cl(-) co-transport.

Pacemaking in interstitial cells of Cajal depends upon calcium handling by endoplasmic reticulum and mitochondria

Pacemaker cells, known as interstitial cells of Cajal (ICC), generate electrical rhythmicity in the gastrointestinal tract. Pacemaker currents in ICC result from the activation of a voltage-independent, non-selective cation conductance, but the timing mechanism responsible for periodic activation of the pacemaker current is unknown. Previous studies suggest that pacemaking in ICC is dependent upon metabolic activity 1y1yand1 Ca2+ release from intracellular stores. We tested the hypothesis that mitochondrial Ca2+ handling may underlie the dependence of gastrointestinal pacemaking on oxidative metabolism. Pacemaker currents occurred spontaneously in cultured ICC and were associated with mitochondrial Ca2+ transients. Inhibition of the electrochemical gradient across the inner mitochondrial membrane blocked Ca2+ uptake and pacemaker currents in cultured ICC and blocked slow wave activity in intact gastrointestinal muscles from mouse, dog and guinea-pig. Pacemaker currents and rhythmic mitochondrial Ca2+ uptake in ICC were also blocked by inhibitors of IP3-dependent release of Ca2+ from the endoplasmic reticulum and by inhibitors of endoplasmic reticulum Ca2+ reuptake. Our data suggest that integrated Ca2+ handling by endoplasmic reticulum and mitochondria is a prerequisite of electrical pacemaking in the gastrointestinal tract.

Effects of removal and reapplication of K(+) and Cl(-) on spontaneous electrical activity, slow wave, in the circular muscle of the guinea-pig gastric antrum

In the circular muscle of guinea-pig gastric antrum, the effects of removal and reapplication of K(+) and Cl(-) were studied on the slow wave, which consists of the lower, first and upper, second components. The first component appeared to be triggered by the driving potential generated in the interstitial cells. K(+) removal slightly depolarized the membrane, increased frequency, and shortened the first component and driving potential, and K(+) reapplication hyperpolarized and prolonged these potentials transiently. Ouabain abolished the K(+)-induced hyper-polarization but had no inhibitory effect on the K(+)-induced potentiation. The K(+)-induced prolongation was much reduced in Ca(2+)-deficient and increased in Ca(2+)-excess solution. BAPTA-AM, thapsigargin, and cyclopiazonic acid shortened the slow wave and inhibited the K(+)-induced prolongation but did not block the slow wave. Effects of Cl(-) removal were stronger than K(+) removal in shortening and increasing the frequency. In Cl(-)-deficient solution, no prolongation was observed on K(+) reapplication. Although no conclusive evidence was obtained as to the ionic mechanism involved in the effects of K(+) or Cl(-) removal and reapplication, a possibility is considered that the sarcoplasmic reticulum is involved in determining the duration of the driving potential and the first component of the slow wave.