Quinidine blockade of the carbachol‐activated nonselective cationic current in guinea‐pig gastric myocytes

Abstract

1 In guinea-pig gastric myocytes isolated from the antral circular layer, stimulation of muscarinic receptors by carbachol (CCh) induces a cationic current (ICch) which is known as the main mechanism of depolarization induced by muscarinic stimulation. 2 We tested the effects of a number of ion channel blockers on ACCh and focused upon quinidine which was a highly potent blocker. Externally applied quinidine suppressed ICch (IC50=0.25 JM) in a reversible and voltage-dependent manner. Applied internally, quinidine was about 100 times less potent than when applied externally. Persistent activation of G-protein by GTPyS also induced a cationic current similar to ICch and this current was also blocked by quinidine. 4-Aminopyridine and tetraethylammonium also suppressed ACCh in a dose-dependent manner (IC50=3.3 mM and 4.1 mM, respectively). 3 Pretreatment with quinidine (2 pM) selectively blocked the acetylcholine (ACh)-induced depolarization which was recorded in the multicellular tissues by a conventional intracellular microelectrode technique. 4 Voltage-dependent K-currents were also suppressed by quinidine but in a higher concentration range (IC50= 3 gM). Quinidine, 10 gM, decreased the amplitude of the voltage-dependent Ca current to only a small extent (15% decrease at 0 mV). Quinidine, 2 gM, also suppressed only a minute proportion of the Ca-activated K current (11.1% decrease at 45 mV). 5 From these experiments, it is concluded that some organic agents known as K channel blockers are able to block the CCh-activated cation channel in a non-specific manner and among them, quinidine can be used as an effective blocker for Acch in guinea-pig gastric myocytes.

Effects of inhibitors of nonselective cation channels on the acetylcholine-induced depolarization of circular smooth muscle from the guinea-pig stomach antrum

“…These chemicals are known to inhibit muscarinic receptor coupled non-selective cation channels (quinidine, Ni 2+ , flufenamic acid), Ca 2+ -sensitive Cl --channels (flufenamic acid, niflumic acid, DIDS) or capacitative Ca 2+ channels (Ni 2+ , La 3+ , SKF-96365) (Kuriyama et al, 1998), suggesting that any one of these channels is important for ACh-induced depolarization. Thus, these results did not support the conclusion obtained in patch clamp experiments using dispersed single smooth muscle cells (Inoue and Isenberg, 1990;Chen et al, 1993;Kim et al, 1995;Zholos and Bolton, 1995;So and Kim, 2003). Although the discrepancy between the results reported previously and those presented here remains unclear, the following two possibilities are considered.…”

“…All of these can be mimicked by stimulation of muscarinic receptors with exogenously applied acetylcholine (ACh) or related muscarinic agonists (Bolton, 1978). In single smooth muscle cells or myocytes prepared from gastrointestinal tissues using enzymatic treatment, ACh activates G-protein coupled non-selective cation channels to produce depolarization (Benham et al, 1985;Inoue and Isenberg, 1990;Vogalis and Sanders, 1990;Kim et al, 1995;Kuriyama et al, 1998;Kim et al, 2003;Lee et al, 2003b;So and Kim, 2003). The activation of these channels is inhibited by a group of K-channel blockers such as quinidine, TEA and 4-aminopyridine (Chen et al, 1993;Kim et al, 1995) (Inoue, 1991;Lee et al, 1993;Zholos and Bolton, 1995).…”

Relaxant Effect of Spermidine on Acethylcholine and High K+-induced Gastric Contractions of Guinea-Pig

“…In general, it shifts the Ca 2+ sensitivity of RyR to lower concentrations and releases Ca 2+ in intracellular space (Nagasaki & Kasai M, 1984). Experimentally, transient increase of [Ca 2+ ]i by extracellular application of caffeine has been regarded as an evidence for the presence of CICR in many kinds of cells (Kim et al, 1998). As shown in Fig.…”

Phospholipase C Involvement in Activation of the Muscarinic Receptor-Operated Cationic Current in Guinea Pig Ileal Smooth Muscle Cells

“…The amplitude of I cat immediately before U73122 application (492 ± 151 pA, n = 9) decreased to the value of 84 ± 12 pA (n = 9) at a time of 7 min after its application, with the percent reduction of 77 ± 4% (n = 9). The remaining current at that time was abolished upon application of 10 m M quinidine, a potent I cat blocker (30). The inhibitory effect of U73122 was not reversed at least for 5 min after it was washed away.…”