The rabbit colon displays a diversity of form and function along its proximo-distal axis. Morphologically, four regions can be discerned based on macroscopic and microscopic criteria: 1) the initial portion of the colon immediately distal to the cecum (P1), in which wart-like protrusions characterize the surface topography, is 10 cm in length and endowed with three teniae. 2) The adjoining portion of the colon (P2) possesses one tenia, is about 20cm in length and also displays the wart-like protrusions in slightly less prominent form. 3) Fusus coli, a short segment approximately 4 cm in length, is free of teniae, but exhibits longitudinal folds on its inner aspect. Electron microscopically, it shows a paucity of microvilli in direct contrast to the two afore-mentioned regions. These three portions together constitute the proximal colon. 4) The fourth region of the colon, the distal colon, reaches a length of 80-100 cm and shows no obvious second-order enlargements of its surface, displaying scanning electron microscopically ridges in looped configurations. Physiological parameters also showed differences depending on the region of the colon observed. Water content of the ingesta increases slightly during passage of the proximal colon, decreasing in the fucus coli and distal colon. Na concentration was highest in the area P1-P2, decreasing distally. K was low in area P2 and then rose toward the fusus, only to fall again distally. Nitrogen values decrease considerably during passage of P2 but only slightly distally. Transmural electrical potential differences also exhibit a characteristic, discontinuous gradient.

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Morphologically the large intestine of the nutria resembles that of other caviomorphs, notably the guinea pig. The cecum is voluminous: it contributes 8.6% of the total intestinal length and 12.7% of the total intestinal surface area (considering the surface enlargement factor). It contains 27-32% of the wet ingesta and 20-23% of the dry matter in the gastrointestinal tract. In the colon the corresponding figures are: 21.8% of length, 12.6% of surface area, 16-21% of wet ingesta, and 16-40% of dry matter. The colon can be subdivided both structurally and functionally into two sections, the proximal and the distal colon, the border between the two being the apical flexure of a long parallel loop. The proximal colon (42% of colonic length) displays on the mucosal surface of its mesenterial side a narrow furrow bordered by ridges, which is absent in the distal colon. The ridges contain subepithelial accumulations of coiled tubuloalveolar mucoid glands, entwined by bundles of muscle fibers. Determinations of nitrogen in the contents near the furrow suggest a concentration of bacteria in this part of the lumen. It is hypothesized that the structural differentiations of the proximal colon provide mechanisms for the transport of bacteria from the proximal colon back into the cecum to maintain the fermentation function. The slopes of the longitudinal profiles for dry matter and for concentrations of sodium, potassium and calcium in the luminal contents change at the tip of the parallel loop. The electrical potential difference "intestinal lumen - blood" is particularly large in the proximal colon, indicating active electrogenic ion transport in this region.

Electrical potential difference, short circuit current, tissue conductance, and unidirectional Na-fluxes were measured in four segments of the rabbit large intestine in vitro. Compared to the relatively tight, low conductance distal colon, caecum and proximal parts of the colon are leaky epithelia with high conductances. Net Na-absorption was highest in caecum, and then decreased gradually towards the distal colon, whereas potential and short circuit current where high in the caecum and proximal colon, low in the middle part of the colon, and high again in the distal colon. Unidirectional Na-fluxes of all four segments were different. The discrepancy between the short circuit current and net Na-absorption in the two segments of the proximal colon indicates electrogenic transport of other ions. 0.1 mM ouabain virtually abolished short circuit current and Na-absorption in all segments, whereas 0.1 mM amiloride was not effective in the caecum and the proximal colonic parts. The present study focuses on the comparative aspects of Na-transport. It demonstrates the marked segmental heterogeneity of the basic electrical properties and suggests four different segmental organizations of large intestinal electrolyte transport.

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Without a zeitgeber the circadian rhythms of five physiological functions free-ran with a period length greater than 24 h. Restricted feeding time (RF) masked the free-running rhythms. In addition to masking, entrainment with RF occurred. This process was most evident in locomotor activity and visits to the food box. RF thus had zeitgeber properties in these rabbits. However, in most rabbits the RF zeitgeber was not strong enough to entrain the circadian rhythm completely. A small component free-ran during RF. Following return to continuous food access the whole circadian rhythm resumed to free-run again. In some animals its phase was determined by the RF zeitgeber and in others by the small free-running fraction present during RF. The results suggest that in addition to the light-dark-entrainable circadian oscillator system a feeding-entrainable oscillator exists that takes over phase control of the majority of the rhythm during RF.

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