REGULATION OF INDIVIDUAL NEPHRON FILTRATION RATES

Our long-term objective is to increase our understanding of the regulation
of nephron function by exploiting the unique renal structure of birds, the
highly specialized renal structure of certain desert rodents, and the
specialized transport properties of the nephrons of reptiles. The avian
kidney has a population of nephrons resembling reptilian nephrons that
function independently of each other, do not contribute directly to the
concentrating mechanism, and can cease filtering altogether under some
circumstances and a population of nephrons resembling mammalian nephrons
that function together in the concentrating mechanism, do not normally
cease filtering, but may alter their filtration rates. The kidneys of
certain tiny desert rodents have juxtamedullary nephrons that are several
times larger than the superficial cortical nephrons and a very long papilla
and represent an intermediate state between the avian kidney and the more
commonly studied mammalian kidneys. In contrast to mammalian nephrons,
reptilian and avian nephrons secrete phosphate under the control of
parathyroid hormone. Our primary goals under this broad objective involve
studies of: 1) the regulation of nephron function, including the
regulation of single nephron filtration rates (SNGFR) and tubular
transport, in avian, desert rodent, and reptilian kidneys; 2) the
relationship of single nephron function to the concentrating and dilut ng
mechanism; and 3) the patterns and regulation of renal portal flow within
the avian kidney. We are performing and plan to continue micropuncture and
microperfusion (both in vivo and in vitro) studies of specific factors
regulating SNGRFs (e.g., sodium, chloride, antidiuretic hormone, and distal
tubule-glomerular feedback) and of factors influencing tubular transport,
including phosphate transport (e.g., parathyroid hormone). We also plan
micropuncture studies of loop of Henle function and the effects of
alterations in SNGFR on this function. Electron microprobe analysis is
being used to permit measurement of multiple inorganic ions in the tiny
samples obtained. We also plan in vivo studies of avian renal portal blood
flow patterns by silicone injection techniques and micropressure
measurements and in vitro studies of specific factors (e.g., antidiuretic
hormone) regulating the renal portal valve.