Alterations in colonic smooth muscle function in cats with idiopathic megacolon.
OBJECTIVE
To determine whether colonic smooth muscle dysfunction is involved in the pathogenesis of idiopathic megacolon in cats.


METHODS
In vitro smooth muscle mechanical measurements.


METHODS
Colon from healthy cats and cats with idiopathic megacolon.


METHODS
Colonic smooth muscle strips were suspended in physiologic buffer solution, attached to isometric force transducers, and contracted with acetylcholine (ACh; 10(-9) to 10(-4)M), substance P (SP; 10(-10) to 10(-6)M), cholecystokinin (CCK; 10(-11) to 10(-8)M), potassium chloride (KCl; 10 to 80 mM), or electrical field stimulation (EFS; 25 V, 1 to 30 Hz, 0.5-millisecond duration). Isometric stress responses were compared with those obtained from healthy controls. Colonic smooth muscle strips were also evaluated histologically for neuronal and smooth muscle cell morphology.


RESULTS
Passive isometric stress was not altered, but the active isometric stress responses of megacolon smooth muscle to ACh, SP, CCK, KCl, and EFS were significantly (P < 0.05) diminished, compared with healthy controls. Differences were observed in longitudinal and circular smooth muscle from proximal and distal portions of the colon. Histologic evaluation revealed few abnormalities of smooth muscle cells or of myenteric or submucosal plexus neurons. The contractile response of megacolon smooth muscle to EFS, and the inhibition of this response by tetrodotoxin, suggest that myenteric and submucosal plexus neurons in megacolon smooth muscle are functional.


CONCLUSIONS
Idiopathic megacolon is a generalized dysfunction of colonic smooth muscle in cats. The diminished isometric stress responses to receptor occupancy (ACh, SP, and CCK) and membrane depolarization (KCl) further suggest that the disorder involves disturbance in the activation of smooth muscle myofilaments.
Washabau
R J
RJ
Department of Clinical Studies, School of Veterinary Medicine, University of Pennsylvania, Philadelphia 19104-6010, USA.
Stalis
I H
IH
D000109:Acetylcholine / Q000494:pharmacology; D000818:Animals; D002371:Cat Diseases; D002415:Cats; D002766:Cholecystokinin / Q000494:pharmacology; D003082:Colectomy; D003106:Colon / Q000187:drug effects / Q000502:physiology / Q000503:physiopathology*; D004558:Electric Stimulation; D066298:In Vitro Techniques; D007537:Isometric Contraction / Q000187:drug effects; D008531:Megacolon / Q000503:physiopathology / Q000601:surgery / Q000662:veterinary*; D009130:Muscle, Smooth / Q000187:drug effects / Q000502:physiology / Q000503:physiopathology*; D011189:Potassium Chloride / Q000494:pharmacology; D012016:Reference Values; D013373:Substance P / Q000494:pharmacology
D013373:Substance P; D011189:Potassium Chloride; D002766:Cholecystokinin; D000109:Acetylcholine