Summary of Discussions

It was suggested that the enhanced incorporation of choline into phospholipids in the very first stages of renal compensatory hypertrophy might result in a change of membrane fatty acid composition and of membrane fluidity (S. Solomon). L. M. Lowenstein stated that no data on fatty acid composition of kidneys undergoing compensatory hypertrophy were available. Since ligation of the ureter of the contralateral kidney is known to induce the same compensatory adaptation of tubular reabsorption as contralateral nephrectomy, while, on the other hand, R.A. Malt showed that ligation ofthe contralateral ureter is not equivalent to removing the opposite kidney in respect to RNA accumulation, the role of accelerated phospholipids synthesis in either compensatory adaptation or hypertrophy could be assessed by measuring 14C-choline incorporation in animals with a ligated contralateral ureter (G. Peters). Such experiments, however, have not yet been done (L. M. Lowenstein). H. G. Preuss was asked whether the discrepancy between the 40 percent increase in thymidine incorporation under the influence of "renotropic" factors measured in vitro and the very low mitotic activity of renal cells during compensatory hypertro-phy observed by R.A. Malt could be explained by changes of the cellular thymidine pool (B. Rossier). In reply, H. G. Preuss stated that the pools were not measured but from design had been assumed to be very similar at the start of incubation and that, on the other hand, the transport of 3H-thymidine and of 14C-uridine were unchanged by any of the conditions studied. A. J. Ouellette was asked whether the kidneys were a suitable tissuefor measuring changes in concentrations of different types of RNA, since many different cell types are involved (L. M. Lowenstein). He replied that one hour after injection of3H-orotic acid 98 percent of the label was in the tubular cells and 80 percent in the proximal tubules so that orotic acid labelled selectively a certain cell population. He had, however, no autoradiographic data for (3H-methyl)-methionine labeling. SYNOPSIS The earliest events occurring in remaining renal tissue after loss of renal mass are an accelerated incorporation of choline into phospholipids (5 minutes), possibly an enhanced rate ofsynthesis of cyclic GMP (10 minutes) according to S. E. Dicker (but found to occur later by Schlondorif and Weber) and an increase of a heterodisperse precursor of messenger RNA in the nucleoplasm of renal cells (10 minutes). It is not known how these three types of early events are connected to each …