Fig. 8.
Comparison of the kinetics of adenosine and ATP responses. Application of ATP (200 μm) (light line) from one barrel of a drug pipette elicited outward current responses that were qualitatively and quantitatively similar to those elicited by the ejection of adenosine (also 200 μm) (heavy line) from the adjacent barrel. The only period during which there was a statistically significant difference between these two averaged response waveforms, as determined by lack of overlap in their associated 95% confidence limits, is during the segment bracketed by the vertical lines. For the adenosine response, the raw data were fit by the product of two exponential functions, as described in Materials and Methods. For the ATP response, a somewhat modified function was used. It was assumed that ATP had no direct effect on the receptor, that a constant fraction of the ATP that was present was converted to adenosine per unit time (i.e., first order kinetics), and that the time course of the response to adenosine formed from ATP was governed by the kinetic parameters corresponding to the best fit to the direct response to adenosine. When theT1/2 for the conversion of ATP to adenosine was allowed to vary as a free parameter, the best fit to the ATP response was obtained with a T1/2 of 170 msec. The fit lines corresponding to these two functions are thesmooth lines superimposed on the responses. Time of drug ejection is indicated by the vertical arrow.