Figure 2.

The importance of average intracellular Ca2+ concentration ([Ca2+]i) in the control of systolic (syst) and diastolic [Ca2+]i. A, The effects of application of caffeine (caff) and stimulation rate on [Ca²⁺]_i. A rat ventricular myocyte was stimulated at the frequencies shown above and caff (1 mmol/L) applied for the periods denoted by the red bars. The gray trace is the original data and the black denotes average [Ca²⁺]_i. B, Specimen, averaged traces of [Ca²⁺]_i from the frequencies shown above. For each frequency, the control (black) and caff (red) traces are superimposed. Data reproduced from Sankaranarayanan et al.¹⁴ C, Illustration of flux balance in control and with depressed sarcoplasmic reticulum (SR) function. The total Ca²⁺ efflux via sodium-calcium exchange (NCX) above control diastolic levels is represented by the area under the [Ca²⁺]_i trace. In the depressed SR case, this is separated into 2 components: (1) activated by the syst Ca²⁺ transient and (2) activated by increased diastolic [Ca²⁺]_i. Average [Ca²⁺]_i is identical with normal and depressed SR (A), and, therefore, Ca²⁺ efflux is unchanged and equal to influx.