INTRODUCTION Dietary supplementation with polyunsaturated fatty acids (PUFAs) has been reported as a potential anti-osteoporosis strategy which increases bone calcification in animals. 1 The mechanism by which PUFAs achieve this effect has not been elucidated: active transport in the duodenum may be affected by these agents. 2 Active transport processes are supported by several ion pumps which require ATP(ATPases) located in enterocyte basolateral membranes (BLM). Na + , K + - ATPase is the primary driving force of many transport processes (for example those of monosaccharides) 3 whereas Ca 2+ - ATPase promotes calci- um uptake. 4 The presence of a distinct Mg 2+ - ATPase which could have several possible functions has also been shown in rat duodenum. 5 PUFAs could modulate transport processes in various ways: for example, by changing the lipid profile of the membrane 2 ; by modulation of steroid-induced protein synthesis 6 ; by being converted to an eicosanoid with several possible actions 7,8 ; by action on phospholipases A and C 7 or G proteins 9 ; and lastly by direct activation of protein kinases which would result in phosphorylation of membrane proteins. 7,8 These mechanisms could all play a role in the modulation of membrane ATPases. 10–13 This study describes the action of arachidonic acid (AA), a PUFA of the n-6 series, on Mg 2+ - ATPase activity in BLM of rat enterocytes. It also investigates the possible involvement of protein kinase C in this process. MATERIALS AND METHODS Reagents AA, calphostin (CP), ouabain, ethyleneglycolbis-(b- aminoethyl ether) N,N’-tetraacetic acid (EGTA), phenyl- methyl-sulphonyl fluoride (PMSF) and ATP disodium salt were purchased from Sigma Chemical Co., St Louis, MO, USA. Bisimidolylmaleimide (BI) came from Boehringer- © 2000 Harcourt Publishers Ltd Prostaglandins, Leukotrienes and Essential Fatty Acids (2000) 62(3), 183–187 Inhibition of duodenal enterocyte Mg 2+ -ATPase by arachidonic acid is not mediated by an effect on protein kinase C M. Haag, F. Leonard, O. N. Magada, M. C. Kruger Department of Physiology, University of Pretoria, PO Box 2034, Pretoria 0001, South Africa Summary Active absorption processes in the duodenal enterocyte are driven by various ATPases. It is known that the activity of Na + ,K + -ATPase, Ca 2+ - ATPase and Mg 2+ -ATPase can be modulated by polyunsaturated fatty acids of the n-6 series, for example by linoleic and gamma-linolenic acids. These effects may be achieved by protein phosphorylation via protein kinase C. The present study was undertaken to determine the effect of arachidonic acid on Mg 2+ -ATPase (measured colorimetrically) activity in basolateral membranes prepared from rat duodenum. It shows, for the first time, significant dose-dependent inhibition of Mg 2+ - ATPase (26–62%) by arachidonic acid (10–50 μg/ml) which already takes place after one minute of exposure, indicating involvement of a rapid signal transduction mechanism. Addition of the protein kinase C inhibitors bisimidolylmaleimide (2.5 μM) and calphostin (0.5 μM) did not influence the action of arachidonic acid on Mg 2+ - ATPase; protein kinase C involvement in this process is thus not indicated. © 2000 Harcourt Publishers Ltd Received 26 November 1999 Accepted 6 January 1999 Correspondence to: M. Haag, Department of Physiology, University of Pretoria, PO Box 2034, Pretoria 0001, South Africa. Fax: +27 12 321 1679; E-mail: mhaag@postillion.up.ac.za Prostaglandins, Leukotrienes and Essential Fatty Acids (2000) 62(3), 183–187 © 2000 Harcourt Publishers Ltd doi:10.1054/plef.2000.0139, available online at http://www.idealibrary.com on