Caputo C, Gerday C, Lopez J R, Taylor S R, Bolaños P
IVIC, Centro de Biofísica y Bioquímica, Caracas, Venezuela.
J Comp Physiol B. 1998 Dec;168(8):600-10. doi: 10.1007/s003600050182.
Cooling increases the twitch force of frog skeletal muscle (Rana temporaria; Rana pipiens), but decreases the twitch force of tropical toad muscle (Leptodactylus insularis). Action potentials and intramembranous charge movement in frog and toad fibers were slowed identically by cooling. Cooling increased the integral of twitch Ca2+ detected by aequorin in frog fibers (1.4-fold), while also decreasing the peak and slowing the rate of decay. Conversely, cooling decreased the integral (0.6-fold) and the peak of twitch Ca2+ in toad fibers, without affecting the rate of decay. The difference in entire Ca2+ transients may account for cold-induced twitch potentiation in frogs and twitch paralysis in toads. In sustained contractions of toad fibers, cooling markedly decreased maximum force caused by: (i) tetanic stimulation, (ii) two-microelectrode voltage clamp steps, (iii) high [K+], or (iv) caffeine. Maximum force in sustained contractions was decreased moderately by cooling frog fibers. Rapid rewarming and simultaneous removal of high [K+] or caffeine during a sustained contraction, caused toad muscle force to rise towards the value corresponding to the warm temperature. This did not occur after removing high [K+] or caffeine from toad fibers kept in the cold. Transmission electron micrographs showed no relevant structural differences. Parvalbumins are thought to promote relaxation of frog muscle in the cold. The unique parvalbumin isoforms in toad muscle apparently lack this property.
降温会增加青蛙骨骼肌（林蛙；豹蛙）的抽搐力，但会降低热带蟾蜍肌肉（海岛细趾蟾）的抽搐力。青蛙和蟾蜍纤维中的动作电位和膜内电荷移动因降温而同等程度地减慢。降温使水母发光蛋白检测到的青蛙纤维抽搐Ca2+的积分增加（1.4倍），同时也降低了峰值并减缓了衰减速率。相反，降温降低了蟾蜍纤维抽搐Ca2+的积分（0.6倍）和峰值，而不影响衰减速率。整个Ca2+瞬变的差异可能解释了青蛙中冷诱导的抽搐增强和蟾蜍中的抽搐麻痹。在蟾蜍纤维的持续收缩中，降温显著降低了由以下因素引起的最大力：（i）强直刺激，（ii）双微电极电压钳步阶，（iii）高[K+]，或（iv）咖啡因。降温适度降低了青蛙纤维持续收缩中的最大力。在持续收缩期间快速复温并同时去除高[K+]或咖啡因，会使蟾蜍肌肉力上升至对应于温暖温度的值。从保存在寒冷环境中的蟾蜍纤维中去除高[K+]或咖啡因后，这种情况不会发生。透射电子显微镜照片显示没有相关的结构差异。小清蛋白被认为可促进青蛙肌肉在寒冷中的松弛。蟾蜍肌肉中独特的小清蛋白异构体显然缺乏这种特性。
