Fig. 3. Effects of caffeine in partially depolarized malignant hyperthermia–nonsusceptible (MHN) muscle fiber with elevated myoplasmic free calcium concentration ([Ca²⁺]ⁱ). Simultaneous recording, using a double-barreled microelectrodes, of Vm and [Ca²⁺]ⁱin a MHN fiber in swine physiologic solution (left ), 10 mm K⁺solution (middle ), and K⁺10 mm and caffeine 2 mm (right ). In control, Vm was −83 mV and [Ca²⁺]^Iwas 125 nm. The incubation of a MHN muscle fiber in 10 mm K⁺solution induced a partial depolarization (Vm=−72 mV) that was associated with an elevation of [Ca²⁺]ⁱto 282 nm. Exposure of this partially depolarized muscle fiber to caffeine 2 mm while [Ca²⁺]ⁱwas elevated (approximately 2 times control) induced a further release reaching a value of 998 nm. Measurements of Vm and [Ca²⁺]ⁱbefore and after K⁺and caffeine solution was added to the bath were conducted in same muscle fiber. (Left  and right ) The calibration bars for the Ca²⁺-selective microelectrode and the resting membrane potential, respectively, are shown.