Expression of recombinant horseradish peroxidase C in Escherichia coli.

Epidermal growth factor (EGF ) from mouse submaxillary glands is ;I polypeptide o f hi, 00.15 which stimulates cpithelial cell proliferation ;ind which inhibits gastric acid secretion both i t i iii.o I I I a n d i r i ~ i i r o 12. 3 I. Inhibition o f pepsinogen secretion b y k<iF has been observed i t i t i i v 14 I. but this effect could be secondary t o an inhibition of acid secretion. T h e mechanism by which EGF inhibits acid sccrction induced by histamine may involve ;I decrease in the cyclic A M P content of thc parietal cell 131. T h e intention o f this work wiis to investigate whether E G F could also inhibit acid secretion induced by forskolin which acts directly t o x t iva tc adcnyl;itc cyclase. If such an cffcct were found then ;I comparison could be made with the cffcct o f EGF o n forskolin-stimulatctl pcpsinogen secretion. Stomach cells were isolntctl from the rat fundus by pronase digestion a n d intermittent calcium chclation 15 I. T h e accumulation of the weak base aminopyrine was used iis a n index o f acid secretion ( 5 1, and pcpsinogen secretion was determined by measuring the ability o f supernatant samples t o hydrolysc acid-denatured hacmoglobin [ 6 I. Forskolin produced ;I dose-related increase in the aminopyrine ;iccumulation ratio. which was 49 * 6 (mean f S.F M. from I7 batches o f cells) in the presence o f 50 pM-forskolin. Responses to I h, 5 and I0 pM-forskolin were 13. 33 and 5 S ' L of that obtained with 5 0 pM-for4wlin. L G F ( 7 0 0 i i ~ ] only inh i bit etl am inopy rinc accu ni ulat ion st imu Iatecl by 1.6 ;ind 5 pM-torskolin. Similar results were ohtainccl in the ptcwncc of I 0 ,icM-cimctitlinc. uhich wiis acldecl to block the effects o f any endogcnous histamine. should it be present. 'l 'hc inhibitory action of E<;F against stimulation induced by lo^ concentrations of forskolin wis prevented b y the pi-csencc of ~-isobiitylI -methylxanthinc (0.1 n i M ) a n d by prcincuhntion of the cells for 2 h with pertussis t o x i n ( 100 iig/mli. Forskolin also produced ;I tlosc-rel:itcd stimulation ol' pcpsinogcn secretion. but its action was not inhibited by 200 n M t j G t . uhatcvcr the level of the wcrctory response. LIGF ( 7 0 0 I1M) did not affect either biisd aniinopyrine nccumulotion o r lxisal pcpsinogen secretion. An enriched fraction containing greater than XO'% of parietal cells wiis sonicnted a n d was uscd for assay o f adcnylate cyclase 171. Stimulation o f adenylate cyclase activity by 5 p w f o r s k o h was 8 1 f 17 ( t i = 4) pmol cyclic A M P formed/ 1 0 min per 10" cells. This stimulation was unaffected by the presence of 200 nM-EGF. T h e mechanism by which E G F inhibits acid secretion induccd by histamine 131 and by forskolin may be similar, because, in both cases, the action of EGF was prevented by prcincubation with pertussis toxin or by the presence of 3-isobutyl1-methylxanthine. T h e lack o f effect o f E G F on forskolin-stimulatcd adenylatc cyclase activity, and the action of 3-isobutyl1 -methylxanthine as a phosphodiestcrasc inhibitor, suggest that E G F may activate a cyclic A M P phosphodiestcrase in parietal cells. EGF was only a significant inhibitor of secretory activity stimulated by low concentrations o f forskolin. Forskolin is thought to act by incrcasing thc cyclic A M P content o f parietal cclls and activation of cyclic AMP-dependent protein kinase [ 81. Thercforc, E G F may only be an effective inhibitor when the cyclic A M P content o f the parietal cells was relatively low. One explanation is that there may be several cyclic A M P phosphodicsterasc enzymes in thc parietal cell, and that E G F may only activate one phosphodiesterase with a low K,,, for cyclic AMP. At high concentrations of cyclic AMP, the contribution o f this phosphodiestcrase t o thc rate of cyclic A M P breakdown, and therefore any effect o f EGF, would be minimal. Thcrc was n o evidence for a similar action of E G F against forskolin-stimulate~l pcpsinogcn secretion by chief cells.