In the rat, pancreatic amylase and, to a lesser extent, lipase adapt quantitatively to the amount of their respective substrates in the diet by an increase in specific activity and total contents (range of variation, fivefold for amylase and twofold for lipase). Colipase responded to protein intake (r = 0.85, P less than 0.01) and not to lipids provided protein intake was below 3.5 g or above 6.0 g. With this latter amount of protein, a maximal level was obtained, even with 2% lipid in the diet. Between 3.5 and 6.0 g, lipid intake was found to modulate colipase in parallel with lipase. When different types of fat were compared, the degree of saturation was found to have no impact on lipase, colipase, and amylase. Diets containing medium-chain triglycerides (C8-C10) did not maximally increase specific activity and total content of lipase and colipase, whereas they did not repress amylase as much longer chain triglycerides did. With coconut oil (45% C12), lipase was maximally stimulated but amylase was not maximally repressed, showing that the regulation of the hydrolases may be partly reciprocal and partly independent.
The adaptative response of the exocrine pancreas to diets rich in lipids and the influence of dietary proteins are reconsidered taking into account colipase. This was particularly necessary in the rat since colipase is often limiting with respect to lipase in this species. In a first experiment, adult male rats received diets containing 10, 15, 18, 40 or 86% casein together with 2, 8 or 40% lard. Under these conditions, lipase was found to be sensitive to variations in the lipid contents of the diet and was two-fold higher with 40% than with 8% lipid, in diets containing 40% casein. Proteins exerted a permissive effect since no response was recorded with diets containing less than 18% casein. Colipase responded to protein intake, even in lipid-poor diets (2% lard), and was increased by a factor 3 when casein was raised from 18 to 40%. In a second experiment, proteins were fed as a separate meal, and the remainder of the diet (provided ad libitum) contained either 8 or 40% lard. In a diurnal study, lipase and colipase were followed every 3 hours over 24 hours. Both lipase and colipase were found to accumulate after the protein meal. Colipase was found to accumulate much faster than lipase in all cases and reach 3 times basal levels 9 hours after the protein meal. This resulted in important diurnal variations in the ratio of colipase to lipase which modulates lipolytic activity. It is concluded that colipase is particularly sensitive to protein intake, perhaps more than to lipid intake and may become and limiting factor of lipid digestion.
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