Regulation of myogenic tone in cerebral and mesenteric resistance arteries by metabolic agents and second messenger systems
1. The pressure-perfusion myograph. permeabilisation techniques and also intracellular membrane potential recordings were used to examine the regulation of myogenic tone in cerebral and mesenteric resistance arteries by metabolic agents and second messenger systems. 2. After pressurisation to 60 mmHg, rat isolated mesenteric and cerebral resistance arteries developed spontaneous myogenic tone, resulting in a 26 ± 1% (n = 42) and 30 ± 2% (n = 14) reduction in diameter respectively. 3. The metabolic vasodilator adenosine and the KATI' channel opener cromakalim each produced a dose-dependent dilatation of pressurised mesenteric arteries. The cromakalim-evoked dilatation was inhibited by glibenc1amide (l J.lM), demonstrating the presence of the KATI' channel in the mesenteric artery and their activation as the mechanism for cromakalim-evoked dilatation. In contrast neither adenosine nor cromakalim produced a dilatation of pressurised cerebral arteries. 4. Adenosine-evoked dilatation of mesenteric arteries was unaffected by the nitric oxide synthase inhibitor L-NAME (100 IlM). antagonists of the KATI' channel (gJibenclamide; 1 J.lM), the small conductance Ca2' activated K+ channel (apamin; 0.3 J.lM) and the large conductance, Ca2! activated K+ channel (TEA; 1 mM). Further to this, cromakalim (10 IlM) but not adenosine (100 J.lM) produced a hyperpolarisation of the pressurised mesenteric artery. This suggests that neither nitric oxide synthesis nor K+ channel activation contributed to the adenosine-evoked dilatation. 5. Adenosine evoked adose-dependent dilatation of p-escin permeabilised mesenteric arteries; where the intracellular Ca" concentration was clamped to ~600 nM. The mechanism of adenosine-evoked dilatation may involve a decreased myofilament Ca2+ sensitivity. 6. An increase in extracellular potassium ion concentration ([K+lo) may link increased neuronal activity and regional cerebral blood flow. Elevation of [K+Jofrom 4.7 to 10 mM evoked a sustained dilatation of isolated pressurised thalamo-perforating cerebral arterioles. 7. The K+-evoked dilatation was inhibited by the inward rectifier K+ channel (K1R) inhibitor Ba2+ (50J.lM), and the K+ channel inhibitor cesium (20mM) but was not blocked by inhibitors of the ATP-sensitive (KATP)and the Ca2 + -activated K+ channel (KcJ, glibenclamide (l J.lM) and TEA (lmM) respectively. Nor was the dilatation altered with the neurotoxin tetrodotoxin (TTX, 0.3 J.lM). The K+--evoked dilatation was associated with a membrane hyperpolarisation to -58 ± I mV (n = 5), from a control value of -42 ± 1 mV (n = 10). 8. It is proposed that increased [K+Jo evokes a dilatation of thalamoperforating cerebral arteries via an activation of KIR channels and smooth muscle cell hyperpolarisation. 9. An increase in [Ca2+]o to approximately 700 nM evoked a 30 ± 3 % (n = 28) constriction of isolated ~-escin permeabilised cerebral resistance arteries. 10. Under [Ca2+1 clamped conditions the putative PKC activator indolactam evoked a 20 ± 2% constriction of the artery. The PKC inhibitor (PKC(19_ 36); I IlM) produced a near maximal (85 ± 4 %) reversal of the indolactam-evoked constriction of the artery, while PKC(19_36) (1 IlM) produced only a minor (12 ± 3 %) reversal of the Ca2+-induced constriction, thus confirming that the indolactam-evoked constriction was due to an activation ofPKC. 11. The MLCK antagonist SM-l (100 JlM) reversed both the Ca2+_ and the indolactam-evoked constriction of the artery. The calmodulin antagonist RS-20 (0.1 - 100 JlM) dose-dependently reversed the Ca2 + -evoked constriction but, even up to a concentration of 300 JlM, did not reverse the indolactam evoked-constriction of the artery. 12. It is proposed that MLCK but not calmodulin plays a role in the PKCevoked smooth muscle contraction.