PKC ISOFORMS AND OTHER SIGNALING PROTEINS INVOLVED IN SURFACTANT SECRETION IN DEVELOPING RAT TYPE-II CELLS

We previously reported that there is a developmental increase in surfa ctant secretion in response to P2Y(2) purinoceptor agonists. UTP does not stimulate secretion in type II cells from 1- or 2-day-old rats; th ere is a small response to UTP in cells from 4-day-old animals, and th e response increases with increasing age thereafter. Second messenger formation in response to P2Y(2) agonists has a similar developmental p attern. We have investigated whether the failure to respond to P2Y(2) agonists is due to a deficiency in the P2Y(2) receptor or in downstrea m signaling factors. We compared type II cells from adult and 1- to 2- day-old rats with respect to expression of the P2Y(2) receptor gene an d the levels of phospholipase C-beta (PLC-beta) and protein kinase C ( PKC) isomers and of the ol-subunit of the GTP-binding protein G(q). We measured gene expression by reverse transcriptase-polymerase chain re action and protein levels by immunoblotting. We identified PKC-alpha, -beta I, -beta II, -delta, -eta, -zeta, -theta, and -mu, PLC-beta 3, a nd G(q) alpha in adult and newborn type II cells. PKC-epsilon, -gamma, and -lambda and PLC-beta 1, -beta 2, and -beta 4 were not present in adult or newborn type II cells. Expression of the P2Y(2) receptor gene was essentially the same in newborn and adult cells. However, the lev els of PKC-alpha, -beta I, -beta II, and -zeta in newborn type II cell s were only 43-57% those of adult cells. The level of PKC-theta also t ended to be lower in the newborn cells. There was little difference be tween newborn and adult type II cells in the levels of PKC-delta, -eta , and -mu, PLC-beta 3, and G(q) alpha. These data suggest that the lac k of response of early newborn type II cells to P2Y(2) agonists is not due to a lack of expression of the receptor gene but possibly to insu fficient amounts of one or more of the alpha, beta I, beta II, or zeta PKC isoforms.