Sabat P, Novoa F, Bozinovic F, Martínez del Rio C
Departamento de Ciencias Ecológicas, Facultad de Ciencias, Universidad de Chile, Santiago.
Physiol Zool. 1998 Mar-Apr;71(2):226-36. doi: 10.1086/515905.
The adaptive modulation hypothesis posits that the expression of digestive proteins should be modulated in response to intake of their respective substrates. A corollary of this hypothesis suggests that dietary flexibility and digestive plasticity should be correlated. We examined these two hypotheses in two granivorous Chilean birds (Zonotrichia capensis and Diuca diuca) that differ in dietary breadth. D. diuca is a strict granivore, whereas Z. capensis also eats insects. In field-caught birds, the activity of the intestinal dipeptidase aminopeptidase-N was positively correlated with intake of insects in Z. capensis but not in D. diuca. This is the first field documentation of modulation of intestinal enzymes by diet in birds. Intestinal maltase and sucrase activities were not correlated with seed (vs. insect) intake in either species. In the laboratory, captive birds of both species exhibited similar modulation of membrane-bound intestinal hydrolases when fed on synthetic diets of contrasting carbohydrate and protein composition. Maltase, sucrase, and aminopeptidase-N activities were significantly higher in birds fed on the carbohydrate-free than those on the carbohydrate-containing diet. Activities of the three enzymes were positively correlated. Therefore, this increase probably resulted from nonspecific increases of all enzymes resulting from intake of the carbohydrate-free diet. Principal components analysis separating the effect of diet on specific and on nonspecific modulation revealed that diet had a strong effect on nonspecific activity of intestinal enzymes in both Z. capensis and D. diuca. Diet also significantly affected aminopeptidase-N activities when the effect of diet on nonspecific modulation was removed. Birds fed on the carbohydrate-free, high-protein diet had significantly higher specific aminopeptidase-N activities than those fed on the carbohydrate-containing diet. Our results cast doubts on the notion that dietary flexibility and the plasticity of the gut's enzymes are necessarily correlated and on the general validity of the adaptive modulation hypothesis.
适应性调节假说认为，消化蛋白的表达应根据各自底物的摄入量进行调节。该假说的一个推论是，饮食灵活性和消化可塑性应该相关。我们在两种食谷的智利鸟类（海角朱雀和迪卡朱雀）中检验了这两个假说，它们在饮食广度上有所不同。迪卡朱雀是严格的食谷鸟，而海角朱雀也吃昆虫。在野外捕获的鸟类中，肠道二肽酶氨肽酶 - N的活性在海角朱雀中与昆虫摄入量呈正相关，而在迪卡朱雀中则不然。这是鸟类中饮食对肠道酶调节的首个野外记录。在这两个物种中，肠道麦芽糖酶和蔗糖酶的活性与种子（相对于昆虫）摄入量均无相关性。在实验室中，当两种鸟类的圈养个体喂食碳水化合物和蛋白质组成不同的合成饮食时，它们对膜结合肠道水解酶表现出相似的调节。喂食无碳水化合物饮食的鸟类中，麦芽糖酶、蔗糖酶和氨肽酶 - N的活性显著高于喂食含碳水化合物饮食的鸟类。这三种酶的活性呈正相关。因此，这种增加可能是由于摄入无碳水化合物饮食导致所有酶的非特异性增加所致。通过主成分分析分离饮食对特异性和非特异性调节的影响，结果表明饮食对海角朱雀和迪卡朱雀肠道酶的非特异性活性均有强烈影响。在去除饮食对非特异性调节的影响后，饮食对氨肽酶 - N的活性也有显著影响。喂食无碳水化合物、高蛋白饮食的鸟类比喂食含碳水化合物饮食的鸟类具有显著更高的特异性氨肽酶 - N活性。我们的结果对饮食灵活性与肠道酶可塑性必然相关这一观点以及适应性调节假说的普遍有效性提出了质疑。
