Caffeine increases Ca2+ responses at selected hot spots. A, Ca2+ responses elicited by two consecutive 50 msec depolarizations to -20 mV (arrowheads) spaced by 20 sec; holding potential, -70 mV. B, top, In a different cell, the second voltage pulse was preceded by application of 1 mm caffeine (red bar). B, bottom, Fluorescence traces corresponding to hot spots 2 and 4 (dotted lines). Note the strong potentiation induced by caffeine at hot spot 2, but not at hot spot 4.