EVIDENCE FOR PH AS AN IMPORTANT PARAMETER TO CONTROL WHEN STUDYING THE CONTRACTILITY OF ISOLATED RABBIT CEREBRAL-ARTERIES

Isolated rabbit basilar (BA), middle (MCA) and posterior (PCA) cerebral arteries mounted in an isometric Mulvany myograph and stretched to a given level of resting tension, developed a spontaneous slow-rising contraction. This tone presented the main features of a classical myogenic tone since it was not suppressed by repetitive washings, was not dependent on the presence of endothelium and was markedly influenced by the concentration of extracellular calcium. In addition, we observed that the occurrence of myogenic tone was dependent on the pH of the medium buffer. Decreasing pH of the Krebs solution from 7.54 to 6.81 by lowering NaHCO3 concentration from 25 to 5 mM, reduced the amplitude of the myogenic tone developed by the arteries. We investigated the influence of such changes of pH on the contractile response to potassium chloride (KCl) and to 5-hydroxytryptamine (5-HT). We demonstrated that the contractile response to KCl (124 mM) was not affected by the pH of the organ bath whereas the maximum contraction to 5-HT(10 mu M) was significantly affected in BA but not in MCA and PCA. Furthermore, we found that three consecutive concentration-response curves to 5-HT were reproducible when obtained at pH 7.15 and 7.30 for BA and MCA. In PCA, 5-HT-induced responses were reproducible at pH 7.30 whereas the sensitivity of the repeated response curves to 5-HT was reduced at pH 7.15. We also noted a larger variability of the response to 5-HT for the three arteries at this pH. Thus, it appeared that bathing rabbit cerebral arteries in a buffer medium at pH 7.30 is suitable for studying their reactivity to 5-HT. These results highlight the importance of controlling the pH of medium buffer when studying in vitro the reactivity of rabbit cerebral arteries. In addition, they define appropriate conditions to minimize the myogenic tone and to allow vigorous and reproducible contractile responses to 5-hydroxytryptamine.