EDRF suppresses chaotic pressure oscillations in isolated resistance artery without influencing intrinsic complexity

It is now widely recognized that nonlinear oscillatory systems can exhibit simple periodicity, characteristic repetitive patterns of odd and even integral periodicity and specific pathways for the transition to irregular, so-called "chaotic," dynamics. In the present study we have identified such behavior in the highly irregular rhythmic vasomotor activity induced by histamine in isolated rabbit ear resistance arteries, thus suggesting a deterministic rather than random etiology. In this experimental model nonlinearity arises at the level of the vascular smooth muscle cell, since oscillatory behavior was not abolished by endothelial denudation. To quantify the complexity of the responses induced by histamine, we applied the analysis of Grassberger and Proccacia (Physica D 9: 189-208, 1983) to calculate a scaling parameter known as fractal dimension, which estimates the minimum number of control variables participating in the genesis of an irregular time-varying signal. The findings suggest the involvement of at least three such variables, because its average numerical value was generally found to be between 2 and 3. Neither the absolute concentration of histamine employed nor pharmacological manipulation (i.e., stimulation/inhibition) of endothelium-derived relaxing factor (EDRF) activity significantly affected the fractal dimension of the pressure fluctuations, although both influenced their superficial form. Histamine and EDRF consequently do not determine the fundamental interactions responsible for generating the chaotic nature of the responses and may be regarded as permissive and modulatory influences, respectively. The well-known unpredictability of nonlinear systems to perturbation may explain why EDRF can either suppress or enhance rhythmic vasomotor activity in different artery types.