Disruption of the atrial natriuretic peptide (ANP) receptor [guanylyl cyclase-A (GC-A)] gene yields mice with a salt-resistant form of hypertension, raising fundamental questions on the role of ANP in acute regulation of the kidney. Here, we show that water intake, food consumption, stool weight, urine volume, and sodium excretion are not significantly different between wild-type and GC-A null mice on standard rodent chow (0.7% NaCl) or a high-salt diet (8% NaCl). In conscious mice with an indwelling catheter, the infusion of a physiological saline solution containing 4% BSA resulted in a marked natriuresis͞diuresis in wild-type mice but no response in GC-A null animals. When physiological saline was given by gavage, however, the kidney response of wild-type and null mice was equivalent, raising the possibility that the gastrointestinal tract can directly regulate kidney function. However, administration of 0.9% saline through an intraperitoneal route also resulted in equal kidney responses in wild-type and null mice. When 0.9% NaCl lacking protein was infused i.v., wild-type and null mice both responded at the kidney level. Thus, GC-A appears dispensable for regulation of sodium͞water excretion in response to changes in dietary sodium concentration, but likely becomes critical in volume expansions where the isooncotic pressure remains constant, such as head-out immersion or the initial and correctable stages of congestive heart failure.

No abstract

Aims:The chemical/physical environment of groundwater may contribute to the existence of a subpopulation of small-sized bacteria (®lterable bacteria) that fails to be trapped on conventional 0á45 lm-pore-size membrane ®lters during routine bacteriological water quality analyses. Efforts were directed to determining an ef®cient recovery method for detection of such cells. Methods and Results: Individual groundwater supplies in a rural setting were examined by a double membrane ®ltration procedure to determine the presence of heterotrophic plate count (HPC) bacteria capable of escaping detection on conventional pore size (0á45 lm) membrane ®lters but retained on 0á22 lm-pore-size ®lters. Since optimum cultural conditions for recovery of ®lterable bacteria are not well de®ned, initial efforts focused on evaluation of various media (R2A, m-HPC and NWRI) and incubation temperatures (15, 20, 28 and 35°C) for speci®c recovery of ®lterable bacteria. Maximum recovery of small-sized HPC bacteria occurred on low-nutrient concentration R2A agar incubated for 7 d at 28°C. Similarly, identical cultural conditions gave enhanced detection of the general HPC population on 0á45 lm-pore-size ®lters. A 17-month survey of 10 well water supplies conducted with the cultural conditions described above resulted in detection of ®lterable bacteria (ranging in density from 9 to 175 cfu ml )1 ) in six of the groundwater sources. The proportion of ®lterable bacteria in any single sample never exceeded 10% of the total HPC population. A majority of the colonies appearing on the 0á22 lm membrane ®lters was pigmented (50±90%), whereas the proportion of colonies demonstrating pigmentation on the larger porosity ®lters failed to exceed 50% for any of the samples (19±49%). Conclusions: A reliable recovery method was developed for the detection of ®lterable bacteria from groundwater. During a subsequent survey study using this procedure, ®lterable bacteria were detected in a majority of the groundwater supplies examined; however, the density of ®lterable bacteria in any single sample never exceeded 10% of the total HPC population. Identi®cation of randomly selected isolates obtained on the 0á22 lm ®lters indicated that some of these ®lterable bacteria have been implicated as opportunistic pathogens. Signi®cance and Impact of the Study: We have determined the presence of small-sized HPC bacteria in ground water that may go undetected when using standard porosity membrane ®lters for water quality analyses. Further study is needed to assess the signi®cance and possible health risk associated with presence of ®lterable bacteria in drinking water supplies from groundwater sources.

No abstract