Prolonged fenoldopam infusions in patients with mild to moderate hypertension: pharmacodynamic and pharmacokinetic effects

Thirty-three patients with mild-to-moderate essential hypertension received either placebo or fenoldopam, a selective dopamine-1 agonist, by intravenous infusion at a fixed infusion rate ranging from 0.1 to 0.8 microg/kg/min for 48 h during a double-blind, placebo-controlled, randomized inpatient clinical trial. Blood pressure and heart rate were measured every 15 min for 24 h before, during, and 24 h after the 48-h drug infusion. Plasma concentrations of racemic fenoldopam were measured at frequent intervals during and for 24 h after fenoldopam infusion. In the 26 patients who received fenoldopam, there were dose-dependent reductions in systolic and diastolic blood pressure, which usually reached a nadir within 2 h of beginning infusion and were significant even at the lowest dose studied (-9 and -9 mm Hg for systolic and diastolic blood pressure, respectively, at 24 h for the dose of 0.04 microg/kg/min, P < .05). There were associated increases in heart rate that were greater in the first than in the last 24 h of drug infusion. Compared to the average 24-h control blood pressure, maximum mean reductions in systolic and diastolic blood pressures of 33 and 21 mm Hg, respectively, were noted in patients receiving fenoldopam at 0.8 microg/kg/min and occurred 4 and 1 h, respectively, after beginning infusion. Tolerance to the blood pressure lowering effects of the drug developed slowly during the 48 h of drug infusion; the half-life for this effect was 60 h. No serious adverse clinical effects were noted in any patient. These results demonstrate that fenoldopam is effective in reducing blood pressure of patients with mild-to-moderate hypertension at doses as low as 0.04 microg/kg/min, is well tolerated at doses up to 0.8 microg/kg/min, maintains most of its antihypertensive efficacy throughout 48 h of continuous, constant rate infusion, and produces neither prolonged pharmacodynamic effects nor rebound hypertension when discontinued. The pharmacodynamic effects of the drug are best predicted by pharmacokinetics of racemic and R-fenoldopam.

Sustained hemodynamic effects of the selective dopamine-1 agonist, fenoldopam, during 48-hour infusions in hypertensive patients: a dose-tolerability study

Eight patients with stage I-II hypertension received a continuous IV infusion of the selective dopamine-1 agonist, fenoldopam, for up to 48 hours at rates from 0.4 to 1.9 micrograms/kg/min. Hemodynamics and clinical symptoms during infusion were compared to the same parameters in the 24-hour periods before and after infusion. Fenoldopam lowered blood pressure and increased heart rate. Greatest changes occurred during the first 12 hours of infusion and gradually returned toward preinfusion values throughout the remaining 36 hours in the six patients who completed 48 hours of infusion. Fenoldopam was discontinued within 2 hours of starting the infusion in two patients who received drug rates of 0.9 microgram/kg/min and 1.9 micrograms/kg/min because of precipitous bradycardia. Clinical symptoms noted at fenoldopam doses higher than 0.8 microgram/kg/min were headache, dizziness, diaphoresis, nausea and vomiting, and restlessness. In this pilot study, fenoldopam effectively reduced blood pressure in patients with stage I-II hypertension for up to 48 hours, but fixed-dose infusion rates above 0.8 microgram/kg/min were associated with a high frequency of clinically significant and often intolerable adverse effects.

Anion secretion drives fluid secretion by monolayers of cultured human polycystic cells

We have investigated the hypothesis that active anion transport drives fluid secretion by the cystic epithelium in autosomal dominant polycystic kidney disease (ADPKD). We prepared monolayers of a primary culture derived from cystic tissue removed from ADPKD patients. The monolayers were grown on permeant supports, and fluid secretion was initiated by forskolin. The results were compared with those obtained with monolayers of Madin-Darby canine kidney (MDCK) cells, known to secrete Cl-. In the absence of the agonist, ADPKD monolayers absorbed fluid (0.20 +/- 0.02 microliter.cm surface area-2.h-1). Forskolin reversed this to secretion (0.60 +/- 0.03 microliter.cm-2.h-1). Control MDCK monolayers did not transport fluid in either direction, but forskolin induced secretion (0.48 +/- 0.03 microliter.cm-2.h-1). The electrical properties of the monolayers were monitored in Ussing chambers. Forskolin increased the transepithelial potential difference (Vte) of ADPKD monolayers (-0.9 +/- 0.1 to -1.1 +/- 0.1 mV) and the short-circuit current (Isc) (6.6 +/- 0.7 to 9.2 +/- 0.8 microA/cm2). The transepithelial resistance (Rte) fell (156 +/- 9 to 138 +/- 10 omega.cm2). Similar results were obtained with MDCK monolayers. The polarity of Vte and the direction of the Isc are compatible with the hypothesis that active secretion of anion drives fluid secretion. Basolateral application of the Na-K-2Cl cotransporter, bumetanide, reduced forskolin-stimulated fluid secretion by ADPKD monolayers (0.56 +/- 0.05 to 0.28 +/- 0.03), depolarized Vte, and inhibited Isc without affecting Rte. Apical application of the Cl- channel blocker, diphenylamine-2-carboxylate, also inhibited fluid secretion by ADPKD monolayers (0.65 +/- 0.03 to 0.27 +/- 0.02 microliter.cm-2.h-1).(ABSTRACT TRUNCATED AT 250 WORDS)

Transepithelial water permeability in an in vitro model of renal cysts

Renal cysts develop from microscopic tubules and may enlarge progressively several thousandfold. Sustained epithelial proliferation, intracavitary fluid accumulation, and extracellular matrix remodeling are central elements in a multistep process that leads to the formation and enlargement of cysts. MDCK cells suspended within medium-hydrated collagen gels grow to form spherical, monolayered, fluid-filled cysts that enlarge steadily. Vasopressin and other agents that increase intracellular levels of cAMP stimulate the rate of MDCK cyst growth and net fluid/solute secretion when added to defined medium in vitro. In this model, net fluid secretion is the only means by which fluid can accumulate within the cyst cavity. We used this cyst-forming line of epithelial cells to evaluate several membrane transport properties that are important in the coupled movements of solute and water in the process of secretory fluid transport. Individual cysts were microdissected from collagen gels, held by a micropipet in a thermostated chamber, and examined at a high magnification by video microscopy. Transepithelial water flow was initiated by rapidly exchanging the bath medium with hyperosmotic solutions. Net water flux, Jv, determined from the initial rate of decrease in cyst diameter, was proportionate to the transmembrane osmotic gradient of NaCl or raffinose; the reflection coefficient for NaCl was indistinguishable from 1.0. Osmotic water permeability (cm3/cm2/osm/min x 10(-6)) was 739 +/- 99 (N = 11) in medium augmented by an NaCl concentration of 100 mosmol/kg. Hydraulic conductivity (Pt), estimated in control cysts, was 6.8 +/- 0.9 microns/s, a value similar to that of medullary and cortical thick ascending limbs of Henle.(ABSTRACT TRUNCATED AT 250 WORDS)

Renal epithelial fluid secretion and cyst growth: the role of cyclic AMP

Transepithelial fluid secretion has been postulated to account for the accumulation of fluid within hereditary and acquired renal cysts, but no such mechanism has been demonstrated in human kidney epithelium. It is shown here that transepithelial fluid secretion was stimulated by prostaglandin E1 (PGE1), forskolin, 8-Br-cyclic AMP, and 1-methyl-3-isobutylxanthine in polarized monolayers of established renal cell lines (MDCK and rat glomerular epithelial cells) and in monolayer cultures derived from the cyst walls of human autosomal dominant polycystic kidney disease and from epithelial cells of normal human renal cortex. Treatment with cyclic AMP agonists caused the same cells, when dispersed within a gel matrix of type I collagen (Vitrogen), to proliferate and form spherical fluid-filled monolayered cysts. Our findings suggest that increased intracellular cyclic AMP levels may have a critical role in the formation and expansion of hereditary and acquired renal cysts.

Renal epithelial cyst formation and enlargement in vitro: dependence on cAMP

Cysts, a common abnormality of kidneys, are collections of urine-like fluid enclosed by a continuous layer of epithelial cells. Renal cysts derive from nephrons and collecting ducts and progressively enlarge as a consequence of epithelial proliferation and transepithelial fluid secretion. The initiation of cyst formation and the factors that control cyst enlargement are unknown. We used an in vitro model of renal cysts to explore the role of the cAMP signal transduction system in the formation and expansion of cysts. MDCK cells, cultured in hydrated-collagen gel, produced polarized monolayered epithelial cysts when intracellular cAMP was increased by prostaglandin E1, arginine vasopressin, cholera toxin, forskolin, or 8-bromoadenosine 3',5'-cyclic monophosphate. All agonists were potentiated by 3-isobutyl-1-methylxanthine, a nucleotide phosphodiesterase inhibitor. The cell proliferation component of cyst enlargement was accelerated by cAMP agonists, as shown by the increased growth of MDCK cells in subconfluent monolayers. The fluid secretion component, reflected by the transepithelial movement of fluid across polarized monolayers of MDCK cells grown on permeable supports, was stimulated by cAMP agonists in the basolateral medium. Chloride levels were higher in the cyst fluid and the secreted fluid than in the bathing medium. We conclude that the development of MDCK cysts is dependent on cAMP. This signal transduction system may be an important modulator of epithelial cell proliferation and transepithelial fluid secretion in the kidney.

Chemical modification of cell proliferation and fluid secretion in renal cysts

We used an in vitro model, MDCK cyst, to determine the extent to which pharmacologic compounds known to inhibit plasma membrane solute transport mechanisms could alter the enlargement of renal epithelial cysts. Solitary MDCK cells cultured within collagen gel undergo clonal growth to form true epithelial cysts in which a single layer of polarized cells (apex toward lumen) encloses a fluid-filled cavity. Repeated observations by light microscopy were used to quantitate the rate of cyst growth in diameter, and demonstrated that cyst enlargement involved an increase in cell number (proliferation) and a net increase in intracystic volume (fluid secretion). Intracyst pressure was greater than the interstitium (6.7 mm H2O +/- 3.1 SD), indicating that fluid entry was secondary to net solute accumulation. Amiloride and seven amiloride analogs that inhibited to different degrees conductive Na+ transport, Na+-dependent H+ transport and Na+-dependent Ca++ transport reversibly decreased the rate of cyst enlargement. The effectiveness of these agents to retard cyst enlargement correlated with their relative potencies to inhibit Na+-dependent Ca++ transport. Morphologic examination indicated that amiloride and amiloride analogs decreased cell proliferation and fluid secretion to the same degree. Ouabain and vanadate (Na+K,ATPase inhibitors), and L-645,695 (Na+-dependent Cl-/HCO3- inhibitor) potently slowed cyst expansion. In contrast to amiloride and amiloride analogs, these agents caused an unusual degree of cellular stratification within the cyst walls, a finding consistent with the notion that fluid secretion was inhibited to a greater extent then cellular proliferation. We conclude that chemical inhibitors of primary and secondary active solute transport can diminish or halt the enlargement of epithelial cysts in vitro by decreasing the rate of cellular proliferation and/or net fluid secretion.