Figure 9. Relations between the caffeine-induced inward current and the [Ca²⁺]_i.

A, time course of caffeine-induced current (continuous line, inward current plotted upward); dashed line shows the [Ca²⁺]_i signal. Both signals filtered with a 10 Hz low-pass filter. B, plot of peak caffeine-induced current and peak [Ca²⁺]_i. Ca²⁺ release was varied by repeating caffeine exposures before the SR had time to fully reaccumulate Ca²⁺. Data from 3 cells shown each as different symbols. Continuous line show a linear regression of these points. C, plot of the caffeine-induced inward current versus[Ca²⁺]_i throughout a single caffeine exposure. Arrow indicates the rising phase of current and [Ca²⁺]_i. The dotted line represents the calculated Na⁺-Ca²⁺ exchange current using the model of Rasmussen et al. (1990) and is given by the equation:

where K_Na,Ca is a scaling factor (4 × 10⁻⁶) which represents the magnitude of the current for a given set of ionic gradients, V is the membrane potential (−60 mV), concentrations are all in millimolar and I_Na,Ca is in nanoamps for a representative bull frog pacemaker cell. [Na⁺]_i was set to 10 mm (the concentration in the pipette solution) and [Na⁺]_o and [Ca²⁺]_o were set to the bath solution (115 and 2 mm respectively). •, shows the mean and s.e.m. for the peak of the caffeine-induced current and the peak of the caffeine-induced [Ca²⁺]_i signal from 12 cells. The dashed line shows the modified Rassmussen model fitted to the mean of the 12 data points which required an 11-fold increase in the value of K_Na,Ca.