Stiffler D F
Biological Sciences Department, California State Polytechnic University, Pomona 91768.
J Exp Biol. 1995 Apr;198(Pt 4):967-74. doi: 10.1242/jeb.198.4.967.
The frog Rana pipiens takes up Ca2+ against an electrochemical gradient from dilute external solutions that are similar to natural freshwater environments. The influx is dependent upon external [Ca2+] and is saturable. Kinetic analysis yielded a Km of 0.625 mmol l-1 and a Jmax of 38 nmol cm-2h-1. These kinetic variables suggest that both the affinity and capacity are smaller than those for Na+ and Cl- transport in the skin of the same species. They are also smaller than those for Ca2+ transport in fish gill. A significant portion (20-25%) of the Ca2+ entering a frog remains in Ca(2+)-rich layers of the skin, with ventral skin containing about three times as much Ca2+ as dorsal skin. There are seasonal rhythms in Ca2+ exchange: although Ca2+ influx does not vary significantly over the year, efflux is minimal in July, while net flux, which is negative most of the year, appears to be positive in July. Since these fluxes do not include dietary calcium, one cannot conclude that feeding frogs are in negative Ca2+ balance.
牛蛙（Rana pipiens）能从类似于天然淡水环境的稀外部溶液中逆电化学梯度摄取Ca2+。其流入量取决于外部[Ca2+]，并且具有饱和性。动力学分析得出Km为0.625 mmol l-1，Jmax为38 nmol cm-2h-1。这些动力学变量表明，其亲和力和转运能力均低于同一物种皮肤中Na+和Cl-的转运。它们也低于鱼类鳃中Ca2+的转运。进入牛蛙体内的Ca2+有很大一部分（20-25%）留在富含Ca2+的皮肤层中，腹部皮肤中的Ca2+含量约为背部皮肤的三倍。Ca2+交换存在季节性节律：尽管一年中Ca2+流入量变化不大，但7月的流出量最小，而一年中大部分时间为负的净通量在7月似乎为正。由于这些通量不包括膳食钙，因此不能得出摄食牛蛙处于负Ca2+平衡的结论。
