Beyenbach K W, Kirschner L B
Am J Physiol. 1975 Aug;229(2):389-93. doi: 10.1152/ajplegacy.1975.229.2.389.
Magnesium concentration in the urinary bladder of marine teleosts is high and [Na] is low. The inverse relationship may be due to the coupling of Mg secretion to Na reabsorption through a common tubular transport system or coupling may be indirect. Unanesthetized sea-water-adapted Salmo gairdneri were infused with saline or MgCl2, and ureteral urine was collected. Over a wide range of Mg excretion rates, tubular Mg secretion and Na reabsorption show no clear correlation as might be expected from an obligatory tubular Mg/Na exchange. Instead ureteral Mg and Na concentrations and excretion rates are positively correlated. These data are not consistent with the presence of a tubular exchange system. When ureteral urine was allowed to remain in the urinary bladder before being analyzed the inverse relationship between [Mg] and [Na] developed because the bladder reabsorbed Na and H2O, thereby concentrating Mg. Hence, tubular Mg/Na exchanges do not produce the inverse Mg/Na relationship in bladder urine. Instead, Mg is secreted in the nephron, while Na and H2O are reabsorbed from the bladder.
海洋硬骨鱼膀胱中的镁浓度较高，而钠浓度较低。这种相反的关系可能是由于镁的分泌与钠的重吸收通过共同的肾小管转运系统相偶联，或者这种偶联可能是间接的。给未麻醉的适应海水的虹鳟鱼输注生理盐水或氯化镁，并收集输尿管尿液。在很宽的镁排泄率范围内，肾小管镁分泌和钠重吸收并未显示出如强制性肾小管镁/钠交换所预期的明显相关性。相反，输尿管镁和钠的浓度以及排泄率呈正相关。这些数据与肾小管交换系统的存在不一致。当输尿管尿液在分析前留在膀胱中时，镁和钠之间的相反关系就出现了，因为膀胱重吸收了钠和水，从而使镁浓缩。因此，肾小管镁/钠交换并不会在膀胱尿液中产生镁/钠的相反关系。相反，镁在肾单位中分泌，而钠和水则从膀胱中重吸收。
