Modulation of pyruvate dehydrogenase kinase activity in cultured hepatocytes by glucagon and n-octanoate

Abstract

The activity of pyruvate dehydrogenase kinase in extracts of mitochondria from rat hepatocytes cultured for 21 h in medium 199 was increased 2.5-fold by the presence of 55 nM-glucagon and 1 mM-sodium n-octanoate in the culture medium. The change was comparable with that induced in vivo by 48 h starvation. The potential contribution of branched-chain complex to estimates of PDH-complex activity in rat liver mitochondria has been defined.

Studies of the long-term regulation of hepatic pyruvate dehydrogenase kinase

“…The stable enhancement of its activity has been variously attributed to increased specific activity of PDHK [8] or to increased enzyme concentration [9]. The PDHK activity enhancement elicited by starvation can be reproduced over a similar timescale (21-24 h) by the addition of glucagon and a fatty acid (n-octanoate) in cultured hepatocytes [10][11][12]. As the increase in hepatocyte PDHK activity in culture is comparable with that of starvation in i o [10], it has been suggested that increased fatty acid oxidation rates and cAMP concentrations mediate the response to starvation [3].…”

“…The PDHK activity enhancement elicited by starvation can be reproduced over a similar timescale (21-24 h) by the addition of glucagon and a fatty acid (n-octanoate) in cultured hepatocytes [10][11][12]. As the increase in hepatocyte PDHK activity in culture is comparable with that of starvation in i o [10], it has been suggested that increased fatty acid oxidation rates and cAMP concentrations mediate the response to starvation [3]. The effect of starvation for 48 h is reversed within 4-8 h of refeeding [6], over which period hepatic cAMP concentrations decline [13] and fatty acid oxidation is suppressed [14].…”

“…The reversal of the effect of LC\HF feeding by insulin infusion suggested that the enhanced hepatic PDHK activity observed in LC\HF-fed rats might result from an impaired action of insulin. The replacement of 7 % of total dietary fatty acids by long-chain ωk3 fatty acids from marine oil abolished the increase in hepatic PDHK activity evoked by 28 days of (5,8,11,14,17) 11.5p2.6 7.2p2.9 C 22 : 5 (7,10,13,16,19) 0.7p0.5 2.1p0.4 C 22 : 6 (4,7,10,13,16,19) 5.6p1.7 11.6p1.5* LC\HF feeding, but did not improve the hepatic carbohydrate status (as assessed by glycogen storage) or affect plasma glucose or insulin concentrations ( Table 2). A significant effect of longchain ωk3 fatty acid supplementation was achieved within 24 h, after which time hepatic PDHK activities had declined to 0.117p0.017 min −" (n l 6).…”

“…Starvation (48 h) increased hepatic PDHK activity by 3.5-fold [from 0.090p0.010 (n l 13) min −" to 0.315p0.034 min −" (n l 6) ; P 0.001] (see also [5,7,10,15]). Provision of LC\HF diet for 28 days also increased hepatic PDHK activity [by 2.1-fold, to 0.192p0.022 min −" (n l 6) ; P 0.001].…”

“…Methods used for preparation and culture of hepatocytes are described in [10,12]. Culture was for 4 h in medium 199 containing 5 % (v\v) foetal calf serum, followed by 21 h in medium 199 with a change of medium 199 after 17 h. Additions of n-octanoate, dibutyryl cAMP and insulin were made to the culture medium for the final 21 h of culture with the exception of some studies in which it is specified that insulin was included only for the final 4 h of culture.…”