Voltage clamp studies on single cells from bull-frog atrium have been carried out to study the ion transfer characteristics of the calcium current, ICa. In agreement with the preliminary results of Hume & Giles (1983), a TTX-resistant, 'second transient inward current' was recorded consistently. Its average peak size at 0 mV in 2.5 mM [Ca2+]o Ringer solution was approximately -200 pA, and it was blocked by Cd2+ and La3+ but not by tetrodotoxin (TTX, 3 x 10(-6) M). 2. The peak size of this current increases by approximately 4 times when [Ca2+]o is raised from 1.25 to 7.5 mM, indicating that Ca2+ is a major charge carrier. 3. A well-defined reversal potential, Erev, for ICa can be recorded in normal Ringer solution and also when Ba2+ or Sr2+ serve as the charge carriers. When [Ca2+]o is changed the shifts in Erev follow the predictions of a Nernstian Ca2+ electrode. However, all Erev values are well below those predicted from the thermodynamic Nernstian ECa values (see Campbell, Giles, Hume, Noble & Shibata, 1988a). 4. The Ca2+ current exhibits voltage-dependent inactivation, whether the direction of net current flow is inward or outward; however, the rate of inactivation is affected by the species of cation carrying the current. Inactivation is reduced substantially in Ba2+ Ringer solution. 5. Magnesium (5 mM) is not a significant carrier or blocker of ICa in normal [Ca2+]o Ringer solution; however, 5 mM [Mg2+]o can block the current carried by either Sr2+ or Ba2+. In the absence of Mg2+, equimolar substitutions of Sr2+ or Ba2+ for Ca2+ result in larger currents than those carried by Ca2+ in the normal Ringer solution. 6. Sodium appears not to be a significant charge carrier in the presence of normal [Ca2+]o. However, after free [Ca2+]o has been reduced to extremely low levels (less than 10(-6) M) Na+ can carry a significant fraction of 'ICa'. Thus, it appears that the high selectivity of ICa for Ca2+ ions depends upon the presence of Ca2+. 7. 'Slow tails' are frequently recorded after repolarizing clamp steps back to the holding potential. These 'slow tails' are prominent in normal [Na+]o, [Ca2+]o and [Sr2+]o Ringer solution; however, they are markedly reduced in [Ba2+]o, in Na+-free and Ca2+-free Ringer solutions. Experimental and theoretical work suggests these slow tails may be generated by an electrogenic Na+-Ca2+ exchanger (see Campbell, Giles, Robinson & Shibata, 1988b).(ABSTRACT TRUNCATED AT 400 WORDS)
摘要
为研究钙电流（ICa）的离子转运特性，对牛蛙心房的单个细胞进行了电压钳研究。与休姆和贾尔斯（1983年）的初步结果一致，持续记录到一种对河豚毒素（TTX）耐受的“第二瞬时内向电流”。在2.5 mM [Ca2+]o林格溶液中，其在0 mV时的平均峰值大小约为 -200 pA，且被Cd2+和La3+阻断，但不被河豚毒素（TTX，3×10(-6) M）阻断。2. 当[Ca2+]o从1.25 mM升高到7.5 mM时，该电流的峰值大小增加约4倍，表明Ca2+是主要的电荷载体。3. 在正常林格溶液中以及当Ba2+或Sr2+作为电荷载体时，均可记录到明确的ICa反转电位Erev。当[Ca2+]o改变时，Erev的变化遵循能斯特Ca2+电极的预测。然而，所有Erev值均远低于根据热力学能斯特ECa值预测的值（见坎贝尔、贾尔斯、休姆、诺布尔和柴田，1988a）。4. 无论净电流流动方向是内向还是外向，Ca2+电流均表现出电压依赖性失活；然而，失活速率受携带电流的阳离子种类影响。在Ba2+林格溶液中，失活显著降低。5. 在正常[Ca2+]o林格溶液中，镁（5 mM）不是ICa的重要载体或阻断剂；然而，5 mM [Mg2+]o可阻断由Sr2+或Ba2+携带的电流。在无Mg2+的情况下，用Sr2+或Ba2+等摩尔替代Ca2+会导致比正常林格溶液中Ca2+携带的电流更大的电流。6. 在正常[Ca2+]o存在时，钠似乎不是重要的电荷载体。然而，当游离[Ca2+]o降至极低水平（低于10(-6) M）后，Na+可携带相当一部分“ICa”。因此，似乎ICa对Ca2+离子的高选择性取决于Ca2+的存在。7. 在将钳制步骤复极化回到保持电位后，经常记录到“慢尾”。这些“慢尾”在正常[Na+]o、[Ca2+]o和[Sr2+]o林格溶液中很突出；然而，在[Ba2+]o、无Na+和无Ca2+的林格溶液中它们明显减少。实验和理论研究表明，这些慢尾可能由电生Na+-Ca2+交换器产生（见坎贝尔、贾尔斯、罗宾逊和柴田，1988b）。（摘要截断于400字）
