Publications

We evaluated the role of lipoxygenase products of arachidonic acid metabolism in mechanical hyperalgesia induced by epinephrine, an agent that directly sensitizes nociceptors to produce mechanical hyperalgesia via three second messenger signaling pathways, protein kinase A (PKA), protein kinase C epsilon (PKCepsilon), and mitogen activated protein kinase (MAPK). Epinephrine hyperalgesia and that induced by a selective activator of PKCepsilon, psiepsilonRACK, were inhibited by nordihydroguaretic acid (NDGA, non-selective lipoxygenase inhibitor), baicalein (BAIC, 12-lipoxygenase inhibitor) and 5, 6-dehydroarachidonic acid (5, 6-dhAA, 5-lipoxygenase inhibitor). NDGA and 5, 6-dhAA inhibited the hyperalgesia associated with activation of the protein kinase A pathway, elicited by the direct-acting hyperalgesic agent prostaglandin E(2) or by the catalytic subunit of protein kinase A. The hyperalgesia produced by active MAPK was not blocked by any of the lipoxygenase inhibitors. Injection of 5- and 12-lipoxygenase produced hyperalgesia that was not antagonized by inhibitors of PKA, PKCepsilon or MAPK. These findings suggest that: (1). lipoxygenase products of arachidonic acid function as second messengers in the peripheral hyperalgesia induced by agents that act directly on primary afferent nociceptors (epinephrine and prostaglandin E(2)), (2). products of the 5-lipoxygenase and 12-lipoxygenase pathway are involved in this function, and (3). these lipoxygenase products contribute to hyperalgesia at or downstream of protein kinase A and PKCepsilon.

We have shown in previous studies that high extracellular tonicity is associated with increased expression of the type A natriuretic peptide receptor (NPR-A) and reduced expression of the endothelial NO synthase (eNOS) gene in cultured rat inner-medullary collecting duct cells. The vasoactive peptide endothelin has been shown to be avidly expressed in this nephron segment, and to be subject to osmotic regulation. We asked whether endothelin might play a role in the control of basal or osmotically regulated NPR-A or eNOS gene expression in these cells. Although exogenous endothelin had little or no effect on basal expression of eNOS mRNA or protein or NPR-A gene expression, both the type A (BQ610) and type B (IRL1038) endothelin receptor antagonists proved capable of reducing eNOS mRNA and protein expression, and increasing levels of the NPR-A mRNA. Increased extracellular tonicity reduced endothelin mRNA accumulation in these cells (approximately 15% of control levels); however, exogenous endothelin failed to normalize osmotically increased NPR-A activity or expression, or osmotically suppressed eNOS expression. Collectively, these data demonstrate the presence of a number of independent but highly interactive local regulatory networks governing fluid and electrolyte handling in this distal nephron segment.