Action of palytoxin on apical H⁺/K⁺‐ATPase in rat colon

Abstract

Palytoxin stimulated a cation-dependent short-circuit current (Isc) in rat distal and proximal colon in a concentration-dependent fashion when applied to the mucosal surface of the tissue. The distal colon exhibited a higher sensitivity to the toxin. The palytoxin-induced Isc was blocked by vanadate but was resistant to ouabain or scilliroside, suggesting the conversion of a vanadate-sensitive H + /K + -ATPase into an electrogenic cation transporter. Cation substitution experiments with basolaterally depolarized tissues suggested an apparent permeability of the palytoxin-induced conductance of Na + >K + >Li + . Immunohistochemical control experiments confirmed the absence of the Na + /K + -ATPase in the apical membrane. Consequently, the pore-forming action of palytoxin is not restricted to Na + /K + -ATPase but is also observed with the colonic H + /K + -ATPase.Keywords: ATPase; colon; palytoxin; Isc; ion channel. P 2C -type ATPases are oligomeric enzymes consisting of a and b subunits [1]. The sodium pump from the plasma membranes of animal cells, a member of this group, generates a sodium gradient by pumping three Na + ions out of the cell and two K + ions into the cell for each ATP hydrolyzed [2]. This sodium gradient is the driving force of all secondarily active transporters and a presupposition for neuronal conduction of signals.The closest relatives of the sodium pump are the proton pumps from gastric and colon epithelial cells [3]. Although these pumps are not identical, they both catalyze an active secretion of protons driven by ATP hydrolysis. Unlike the sodium pump, however, both proton pumps are electroneutral: each transports one K + ion from the luminal side into the cytosol for each H + secreted.Several naturally occurring toxins have been identified as specific inhibitors of the sodium pump. Among them, the so-called cardioactive steroids or cardiac glycosides are not only known for their ability to selectively target the sodium pump but are widely used as effective medication for patients with heart failure or heart insufficiency [4]. Palytoxin, a toxin isolated from corals of the family Palythoa (e.g. Palythoa caribaeorum), is also a highly specific inhibitor of the sodium pump [5,6]. This most potent toxin (for rodents the LD 50 is 10-250 ng per kg of body weight) of animal origin can also be found together with ciguatoxin, maitotoxin, or gambierol in fishes that contribute to ciguatera poisonings [7,8]. Palytoxin is a rather unique and large molecule with the structural formula C 129 H 223 N 3 O 54 . The molecule can be divided into three subdomains, each connected by peptide bonds: a large N-terminal polyhydroxy x-amino acid followed by a dehydro-b-alanine residue and an aminopropanol group. The number of free hydroxyl groups is 42 [9,10]. Unlike the cardioactive steroids, however, which inhibit both ATP hydrolysis and ion conduction, palytoxin acts by arresting the ionophore of the pump into a permanently open state. Thus, in this case, inhibition of ATP hydrolysis is no longer associated ...

Palytoxin Acts on Na+,K+-ATPase but not Nongastric H+,K+-ATPase

“…There is controversy regarding its pharmacological sensitivity to ouabain (Codina et al 1996;Cougnon et al 1996;Cougnon et al 1998;Sangan et al 2000) and this complicates the interpretation of experiments in which both PTX and ouabain are present. It has been reported that PTX has an effect on both distal and proximal colon, and it has been suggested that this action of PTX is mediated by interaction with ngH,K-ATPase (Scheiner-Bobis et al 2002). However, since the conductance increase produced by PTX action on the Na,K-ATPase is so large, the presence of only a small amount of Na, K-ATPase in distal and proximal colon would be sufficient to explain the results obtained in those tissues.…”

“…Reports on the effect of palytoxin on the apical versus basolateral side of polarized epithelial cells from kidney (LLC-PK1), provide additional evidence that the site of palytoxin action is the Na,K-ATPase (Mullin, et al 1991). However, it has been reported that palytoxin acts on both distal and proximal parts of the descending colon (Scheiner-Bobis, et al, 2002). Extensive pharmacological characterizations of colonic and other H,K-ATPases found in collecting duct have yielded conflicting conclusions regarding its sensitivity to ouabain (Codina J., et al, 1996;Cougnon, et al, 1996;Del Castillo, et al, 1991).…”

Palytoxin disrupts cardiac excitation-contraction coupling through interactions with P-type ion pumps

“…The fact that palytoxin interferes with both the Na ϩ -K ϩ pump and the SERCA might be explained by structural similarities and the high degree of homology between the two ion pumps (48). In line with this notion, Scheiner-Bobis et al (45) suggested in a recent report that palytoxin can convert the colonic H ϩ -K ϩ pump, another closely related P-type ion pump, into a cation channel. While our experiments on isolated SR vesicles clearly establish an inhibitory action of palytoxin on the SERCA, they do not provide any insights into the mechanism of inhibition.…”

Clostridium difficile colitis: A marker for ischemic colitis?

“…4 These drugs stimulate cation-dependent short-circuit currents in the colonic mucosa, possibly by converting the vanadate-sensitive H+/K+-ATPase into an electrogenic cation transporter. 5 Should the demand for energy from ATP (adenosine triphosphate) hydrolysis so induced exceed the capacity for ATP resynthesis, the action might precipitate an aerobic energy deficit or unreversed ATP hydrolysis similar to that developing for different reasons in ischemic colitis that occurs as a complication of abdominal aortic surgery. As in C. difficile colitis, the passage of liquid, bloodstained stools is an established feature of this condition.…”