Figure 1. Effects of isoproterenol on intracellular Ca²⁺ handling.

A, example cytosolic Ca²⁺ transients measured with indo-1 (0.75 Hz, electrical stimuli marked by arrows) followed by rest under control conditions ([Ca²⁺]_o= 2 mm) and in the presence of 1 μm ISO. Control resting [Ca²⁺]_i level is marked by dashed line. Note spontaneous Ca²⁺ wave that occurred during the rest period in the presence of ISO (star). B, example [Ca²⁺]_i increases in response to 10 mm caffeine (black bars) used to assess SR Ca²⁺ content. Caffeine was applied immediately after identical 0.75 Hz pacing trains as shown in A, prior to the occurrence of spontaneous Ca²⁺ waves. Summary data of Ca²⁺ transient amplitude (0.75 Hz; C), resting [Ca²⁺]_i (D) and SR Ca²⁺ load (E) under control conditions, in the presence of ISO, and in the presence of ISO with 7.5 μm cyclopiazonic acid (CPA) in 100 μm[Ca²⁺]_o. *P < 0.05 versus control.