Renal mechanism of trimethoprim-induced hyperkalemia

OBJECTIVES

1) To determine the incidence and severity of hyperkalemia during trimethoprim therapy. 2) To test the hypothesis that trimethoprim inhibits renal potassium excretion by blocking sodium channels in the mammalian distal nephron.

PATIENTS

Thirty consecutive patients who were treated with trimethoprim-containing drugs. All patients included in the study had the acquired immunodeficiency syndrome (AIDS).

EXPERIMENTAL ANIMALS

Thirty-nine male Sprague-Dawley rats receiving normal rat chow and tap water (allowed free access).

INTERVENTION

Humans: high dose (20 mg/kg per day) of trimethoprim therapy. Rats: trimethoprim (9.6 mg/h per kg body weight) was infused intravenously or into the renal distal tubules (1 mmol/L).

MEASUREMENTS

Humans: Serum and urine electrolyte levels, serum creatinine, renin, aldosterone, and cortisol levels were measured, and the transtubular potassium gradient was calculated. Rats: The effects of trimethoprim infusion on urinary sodium, chloride, and potassium concentration and urine volume were measured. Sodium, chloride, potassium, and inulin concentrations were measured in fluid samples obtained from kidney distal tubules. The voltage across the wall of the distal tubule was measured.

RESULTS

Humans: Trimethoprim increased the serum potassium concentration by 0.6 mmol/L (95% Cl, 0.29 to 0.95 mmol/L) despite normal adrenocortical function and glomerular filtration rate. Serum potassium levels greater than 5 mmol/L were observed during trimethoprim treatment in 15 of 30 patients. Rats: Intravenous trimethoprim inhibited renal potassium excretion by 40% (Cl, 21% to 60%) and increased renal sodium excretion by 46% (Cl, 9% to 83%). Trimethoprim (1 mmol/L) in tubule fluid inhibited distal tubule potassium secretion by 59% (Cl, 26% to 92%) and depolarized the lumen-negative transepithelial voltage by 66% (Cl, 46% to 85%).

CONCLUSIONS

Trimethoprim (an organic cation) acts like amiloride and blocks apical membrane sodium channels in the mammalian distal nephron. As a consequence, the transepithelial voltage is reduced and potassium secretion is inhibited. Decreased renal potassium excretion secondary to these direct effects on kidney tubules leads to hyperkalemia in a substantial number of patients being treated with trimethoprim-containing drugs.

Immunocytochemical characterization of the high-affinity thiazide diuretic receptor in rabbit renal cortex

Thiazide diuretics increase urinary NaCl excretion primarily by inhibiting Na and Cl transport across the apical membrane of cells in the renal distal tubule. Although these diuretics bind to a membrane protein that couples transport of Na and Cl directly, the molecular nature of this transporter and its localization in the mammalian kidney remain controversial. The present experiments were designed to develop monoclonal antibodies to the high-affinity thiazide diuretic receptor to investigate its molecular characteristics and its cellular and subcellular localization in rabbit kidney. Mice were immunized with high-affinity thiazide diuretic receptors that had been partially purified from rabbit kidney cortex. Resulting hybridomas were screened for the ability to immunoprecipitate thiazide diuretic receptors that were labeled with the thiazide-like diuretic [3H]metolazone. A single hybridoma (MAb JM5) produced antibodies capable of immunoprecipitating up to 80% of the labeled thiazide receptors from solubilized renal cortical membranes. MAb JM5 reacted with a 125-kDa protein on Western blots of solubilized renal cortical apical membranes. It stained the apical membrane of cells in the distal convoluted and connecting tubule but did not stain proximal tubules, glomeruli, or interstitial structures. Less intense staining of apical membranes of principal cells in the collecting tubule and a subpopulation of cells in the thick ascending limb were also present. These results indicate that the high-affinity thiazide diuretic receptor comprises a 125-kDa protein that localizes to the apical membrane of cells in the renal distal tubule.

Luminal influences on potassium secretion: chloride, sodium, and thiazide diuretics

In the presence of Cl-, K+ secretion by the distal tubule saturates with increasing luminal Na+ concentration. Apparent maximal K+ secretion is attained with luminal Na+ concentrations of 40 mM. The results of the present study show that lowering the Cl- concentration of luminal fluid can increase the level of Na(+)-stimulated K+ secretion beyond the maximal level attained in the presence of Cl-. The effect of lowering luminal Cl- concentration to less than 10 mM on K+ secretion is greater with higher Na+ concentration. Under these conditions, chlorothiazide decreases K+ secretion. When chlorothiazide is present, changing the Na+ concentration does not affect K+ secretion. Because in rats a thiazide effect is attributed primarily to the distal convoluted tubule (DCT), we postulate that it is primarily DCT cells that increase K+ secretion when Na+ concentration is raised in the presence of low luminal Cl- concentration. We propose that the rat DCT cells have both an absorptive Na(+)-Cl- cotransport mechanism and a secretory K(+)-Cl- cotransport mechanism in the luminal membrane that can mediate the apparent exchange of Na+ for K+.

Solubilization and partial purification of the thiazide diuretic receptor from rabbit renal cortex

This study was designed to solubilize, characterize and begin to purify the thiazide-sensitive Na/Cl transporter from mammalian kidney. Metolazone, a thiazide-like diuretic drug, binds to receptors in rat renal cortex closely related to the thiazide-sensitive Na/Cl transport pathway of the renal distal tubule. In the current study, [3H]metolazone bound to receptors in rabbit renal cortical microsomes. The portion of [3H]metolazone binding that was inhibited by hydrochlorothiazide reflected binding to a high-affinity class of receptor. The affinity (Kd 2.0 +/- 0.1 nM) and number (Bmax = 0.9 +/- 0.4 pmol/mg protein) of high-affinity receptors in rabbit renal cortical membranes were similar to values reported previously for rat. When proximal and distal tubule fragments were separated by Percoll gradient centrifugation, receptors were restricted to the fraction that contained distal tubules. When compared with cortical homogenates, receptor density was enriched 12-fold by magnesium precipitation and differential centrifugation. The zwitterionic detergent CHAPS solubilized 25-35% of the receptors (at 6 mM). Chloride inhibited and Na stimulated binding of [3H]metolazone to solubilized high-affinity receptors. The receptors could be purified significantly by hydroxyapatite chromatography and size exclusion high performance liquid chromatography (HPLC). The combination of magnesium precipitation and differential centrifugation, hydroxyapatite chromatography, and size exclusion HPLC resulted in a 213-fold enrichment of receptors, compared to renal cortical homogenate. The current results indicate that thiazide receptors from rabbit kidney share characteristics with receptors from rat, and that rabbit receptors can be solubilized in CHAPS and purified significantly by hydroxyapatite chromatography and size exclusion HPLC.

Adaptation of distal convoluted tubule of rats. II. Effects of chronic thiazide infusion

Mammalian distal tubules adapt structurally and functionally when NaCl concentration in tubule fluid is altered chronically. These experiments were designed to test the hypothesis that chronic administration of hydrochlorothiazide (HCTZ), a drug that blocks Na and Cl uptake across apical membranes of rat distal tubule cells, would reduce intrinsic transport capacity of distal tubules and reduce the number of thiazide-sensitive transporters. Osmotic pumps were implanted into rats to deliver 3.75 mg/day HCTZ or vehicle for 10-14 days. All animals were offered a solution containing 0.8% NaCl and 0.1% KCl as drinking fluid. Free-flow micropuncture after 10-14 days indicated that Na and Cl delivery to distal tubule was not significantly different in HCTZ- and vehicle-treated animals. Microperfusion in vivo with an artificial interstitial solution, with no thiazide, indicated that 10-14 days of HCTZ infusion did reduce Na transport capacity of distal tubules from 390 +/- 32 to 203 +/- 24 pmol/min (P less than 0.01). In contrast, the number of thiazide-sensitive NaCl transporters, determined as high-affinity receptors for [3H]metolazone in renal cortical membranes, was higher in HCTZ group than in controls (2.2 +/- 0.4 vs. 1.0 +/- 0.1 pmol/mg protein, P less than 0.01). These data support the hypothesis that chronic blockade of NaCl entry across apical membranes of distal tubule cells reduces NaCl transport capacity, an effect that occurs despite an increase in the number of thiazide receptors. They indicate that thiazide receptor binding studies should be interpreted in combination with direct functional measurements.

The physiologic basis of diuretic synergism: its role in treating diuretic resistance

Diuretic drugs usually improve edema when used judiciously. Some patients, however, become resistant to their effects. Diuretic resistance may result from dietary indiscretion, poor compliance, impaired bioavailability, imparied diuretic secretion into the lumen of the renal tubule, or because other drugs interfere with diuretic activity. When easily treatable causes of diuretic resistance have been excluded, resistance often reflects the intensity of the stimuli to sodium retention. Recent experimental work has indicated ways in which the kidney adapts to chronic diuretic treatment and has indicated how these adaptations may limit diuretic effectiveness. First, nephron segments downstream from the site of diuretic action increase sodium-chloride (NaCl) reabsorption because the delivered NaCl load increases. Second, diuretic-induced contraction of the extracellular fluid volume stimulates kidney tubules to retain NaCl until the next dose of diuretic is administered. Third, kidney tubules themselves may become hypertrophic because they are chronically stimulated by diuretic-induced increases in NaCl delivery. These adaptations all increase the rate of NaCl reabsorption and blunt the effectiveness of diuretic therapy. When diuretic resistance is present, using a second diuretic drug that acts in a different nephron segment is often effective. Recent experimental results suggest that a second class of drug may act synergistically with the first by blocking the adaptive processes that limit diuretic effectiveness. On the basis of an understanding of the mechanisms of diuretic adaptation and resistance, treatment regimens can be designed to block specific adaptive mechanisms and to improve diuretic therapy.

Luminal calcium regulates potassium transport by the renal distal tubule

We examined the effect of changes in lumen calcium concentration on net potassium transport by distal tubules in anesthetized rats. Tubules were perfused with a control solution that resembled interstitial fluid but lacked calcium. Experimental solutions were prepared by adding varying amounts of CaCl2 to the control solution to produce solutions with free ionic calcium concentration ([Ca2+]) of 0.2, 0.4, and 0.8 mM. In paired comparisons 0.2 mM Ca2+ did not affect net potassium transport, whereas 0.4 and 0.8 mM Ca2+ each reduced potassium secretion by approximately 30%. Unidirectional potassium fluxes using 86Rb as a tracer for potassium and transepithelial voltage (VTE) were measured to characterize further the effect of calcium on potassium transport. Presence of 0.8 mM Ca2+ in the lumen did not affect unidirectional absorptive potassium flux; therefore, the decrease in net potassium flux was accounted for entirely by a decrease in unidirectional secretory potassium flux. The lumen negative VTE measured in the late distal tubule decreased during perfusion with 0.8 mM Ca2+. These results are consistent with the hypothesis that increases in lumen (extracellular) calcium concentration in the range normally present in the distal tubule reduce net potassium secretion by decreasing the electrochemical gradient for potassium secretion.

Adaptation of the distal convoluted tubule of the rat. Structural and functional effects of dietary salt intake and chronic diuretic infusion

We studied the effects of dietary NaCl intake on the renal distal tubule by feeding rats high or low NaCl chow or by chronically infusing furosemide. Furosemide-treated animals were offered saline as drinking fluid to replace urinary losses. Effects of naCl intake were evaluated using free-flow micropuncture, in vivo microperfusion, and morphometric techniques. Dietary NaCl restriction did not affect NaCl delivery to the early distal tubule but markedly increased the capacity of the distal convoluted tubule to transport Na and Cl. Chronic furosemide infusion increased NaCl delivery to the early distal tubule and also increased the rates of Na and Cl transport above the rates observed in low NaCl diet rats. When compared with high NaCl intake alone, chronic furosemide infusion with saline ingestion increased the fractional volume of distal convoluted tubule cells by nearly 100%, whereas dietary NaCl restriction had no effect. The results are consistent with the hypotheses that (a) chronic NaCl restriction increases the transport ability of the distal convoluted tubule independent of changes in tubule structure, (b) high rates of ion delivery to the distal nephron cause tubule hypertrophy, and (c) tubule hypertrophy is associated with increases in ion transport capacity. They indicate that the distal tubule adapts functionally and structurally to perturbations in dietary Na and Cl intake.

Thiazide-sensitive sodium chloride cotransport in early distal tubule

At least two pathways mediate sodium absorption across the luminal membrane of the renal distal tubule. One pathway is a conductive channel and the other appears to be a coupled Na-Cl cotransport pathway. The distal tubule comprises three segments: the distal convoluted tubule, the connecting tubule, and the initial collecting duct. To provide information about cellular locations of the proposed sodium transport pathways, we perfused early (14-38% of whole distal length) and late (61-83% of whole distal length) segments of whole distal tubules separately in vivo in anesthetized rats. When perfused with a solution that resembles fluid normally arriving at the distal tubule (75 mM Na, 68 mM Cl), rates of sodium absorption were similar in early and late segments (early 68 +/- 29.6, late 67 +/- 27.5 pmol X min-1 X mm-1). When perfused with a solution that resembles interstitial fluid (148 mM Na, 110 mM Cl), sodium transport was significantly higher in early than in late segments (276 +/- 28.4 vs. 113 +/- 29.7 pmol X min-1 X mm-1). Chlorothiazide (10(-3) M), which blocks sodium and chloride absorption in whole distal tubules, reduced sodium and chloride transport to zero in early distal tubules but had no significant effect in late distal tubules. Removing all chloride from perfusion solutions reduced sodium transport in early but not late distal segments.(ABSTRACT TRUNCATED AT 250 WORDS)

Chloride-dependent potassium secretion in early and late renal distal tubules

Potassium transport by subsegments of the rat surface distal tubule was studied using a modified in vivo microperfusion method. The nephron segments between 14 and 38% and between 62 and 83% of total distal length distance between macula densa region and confluence of tubule with another) were perfused separately. The first of these two segments is composed primarily of distal convoluted tubule (DCT) cells; the more distal segment is made up primarily by initial collecting tubule (ICT) epithelium. Experiments were performed to measure potassium secretion via two pathways: a diffusion mechanism driven by a favorable electrochemical gradient for potassium, and a cotransport mechanism activated when lumen chloride concentration is low. In a first series of experiments, both the DCT and the ICT secreted potassium when perfused with an artificial control solution resembling fluid normally present at the beginning of the distal tubule. Absolute rates of potassium secretion were higher in the ICT than in the DCT. Decreasing lumen Cl concentration stimulated potassium secretion more in the ICT than in the DCT. In a second series of experiments, the subsegments were perfused with a solution in which ion concentrations were raised to levels found in interstitial fluid. Under these circumstances, potassium secretion was lower in both segments. Decreasing lumen Cl concentration resulted in higher rates of potassium secretion in the DCT than those seen in the first series with low chloride; rates of potassium secretion in the ICT were as high as in the first series.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of calcium infusion on blood pressure in hypertensive and normotensive humans

Disorders of calcium and parathyroid hormone homeostasis have been reported in subjects with essential hypertension. In many of these studies, dietary intakes of sodium and calcium were not carefully controlled. The present study was designed to compare calcium and parathyroid hormone homeostasis in normal and hypertensive subjects on controlled dietary sodium and calcium intakes and to examine the impact of dietary sodium loading on hemodynamic and metabolic responses to infused calcium. Seven subjects with essential hypertension and seven age-matched and sex-matched controls were studied while consuming a standard diet containing 600 mg of elemental calcium. Each subject was studied while consuming 10, 160, and 510 mEq of sodium per day, before, during, and after a 3-hour calcium infusion (3.75 mg/kg/hr). Before calcium infusion, hypertensive subjects had increased urinary cyclic adenosine 3',5'-monophosphate excretion independent of sodium intake (p less than 0.05). Urinary potassium excretion was greater in normotensive than in hypertensive subjects (p = 0.002). At baseline, dietary sodium intake had no effect on systolic, diastolic, or mean arterial pressure. During calcium infusion, systolic pressure increased in both groups, whereas diastolic pressure increased only when dietary sodium content was high and mean arterial pressure increased only in hypertensive subjects (p = 0.007). Together, these data provide evidence for interactions between dietary sodium intake and the cardiovascular response to calcium. They confirm that hypertensive subjects exhibit enhanced parathyroid gland function even when dietary factors are controlled, and they suggest that these subjects are more sensitive to the cardiovascular effects of short-term calcium infusion.

Unidirectional potassium fluxes in renal distal tubule: effects of chloride and barium

Low luminal concentrations of chloride stimulate net potassium secretion by the renal distal tubule, independent of changes in transepithelial voltage. These effects are not prevented by the luminal application of the potassium channel blocking agent barium. Because net potassium secretion comprises secretory and absorptive components, we sought to evaluate the effects of chloride and barium on unidirectional potassium fluxes in the renal distal tubule. In vivo microperfusion methods were used in anesthetized Sprague-Dawley rats. Perfusion solutions contained either 42K or 86Rb as tracers for potassium. Tracer efflux coefficients, indicating apparent potassium permeability, were similar when measured using either isotope. Net potassium flux was determined as the difference between perfusion and collected rate, and unidirectional absorptive potassium flux was calculated as the product of the mean luminal potassium concentration and the tracer efflux coefficient. During perfusion with a solution that resembled fluid normally arriving at the early distal tubule, the absorptive potassium flux was approximately 25% of the unidirectional secretory flux. Reducing lumen chloride concentration increased net potassium secretion, because blood-to-lumen potassium flux increased from 61 +/- 12.7 to 96 +/- 14.6 pmol/min. Barium reduced both absorptive and secretory fluxes but did not prevent the stimulation of net potassium secretion that occurs when luminal chloride concentration is reduced. Apparent potassium permeability during perfusion with a solution that resembled fluid normally arriving at the early distal tubule was 800 nm/s when corrected for voltage. Together with the results of previous experiments, these results are consistent with the presence of a secretory pathway linking potassium with chloride in the luminal membrane of cells of the distal tubule.

Stimulation of distal potassium secretion by low lumen chloride in the presence of barium

Potassium secretion into the renal distal tubule is increased when chloride in the tubule fluid is replaced by another anion. The present experiments were done to determine whether this increment in transported potassium traverses a conductive pathway from cell to lumen. Transport rates of potassium, sodium, chloride, and fluid by the renal distal tubule of rats were examined in vivo by continuous microperfusion. The effects of substituting gluconate for chloride in the presence and absence of 5 mM barium in the perfusion fluid were determined. When gluconate replaced chloride in the perfusion solutions, potassium secretion increased (by 44%) without a significant change in transepithelial voltage. Barium in the lumen increased the magnitude of the lumen-negative transepithelial voltage (by 30%) and reduced potassium secretion (by 56%) by inhibiting conductive potassium movement. Barium also decreased both sodium (by 51%) and chloride (by 37%) absorption. Barium did not reduce the stimulation of potassium secretion caused by reducing lumen chloride concentration. Potassium secretion increased (by 77%) when lumen chloride was reduced in the presence of 5 mM barium. We interpret these results by postulating that a cotransport mechanism linking potassium and chloride is present in the luminal membrane of distal tubule cells, that this mechanism operates in parallel with a conductive transport pathway for potassium, and that the K-Cl cotransport mechanism is not inhibited by barium.

A new class of cardiotonic agents: structure-activity correlations for natural and synthetic analogues of the alkaloid A new class of A new class of cardiotonic agents: structure-activity correlations for natural and synthetic analogues of the alkaloid pumiliotoxin B (8-hydroxy-8-methyl-6-alkylidene-1-azabicyclo[4.3.0]nonanes)

Pumiliotoxin B (PTX-B, 6-(6',7'-dihydroxy-2',5'-dimethyl-(E)-4'-octenylidene)-8-hydroxy-8 -methyl-1- azabicyclo-[4.3.0] nonane) increases the force of contractures of spontaneously beating guinea pig atrial strips by 3- to 5-fold with half-maximal effects at about 3 microM and increases rates of atrial contractions by 2- to 3-fold with half-maximal effects at about 6 microM. The presence of an axial 7-hydroxy substituent (PTX 339A) decreases the efficacy but not the potency of PTX-B as a positive inotropic agent while having only slight effects on activity as a positive chronotropic agent. The presence of an equatorial 7-hydroxy substituent (PTX 339B) greatly decreases efficacy and potency of PTX-B as a positive chronotropic and inotropic agent. Pumiliotoxin A which lacks the side-chain 7'-hydroxy group of PTX-B causes only a 2-fold increase in force of contracture at 54 microM while having minimal effects on rate. The presence of an axial 7-hydroxy substituent (PTX 323B' and 323B", epimeric at the 6'-hydroxy) markedly enhances positive inotropic and chronotropic effects of PTX-A. Another congener, PTX 251D with a 6-(2'-methylhexylidene) side chain, and a synthetic analogue with a 6-(6'-heptenylidene) side chain are cardiac depressants. Both lack hydroxyl groups in the side chain. The presence of an omega-1 hydroxy group in the side chain of PTX 251D yields an alkaloid (267C) with weak positive inotropic effects and minimal chronotropic effects. The presence of an axial 7-hydroxy group in the indolizidine ring of PTX 251D results in a compound (PTX 267A) with very weak positive inotropic effects while retaining the negative chronotropic effects of PTX 251D. A synthetic analogue with a 6-(7'-hydroxyheptylidene) side chain is a cardiac depressant even though it contains a side-chain hydroxyl corresponding in position to the 7'-hydroxyl of the side chain of PTX-B. The positive chronotropic and inotropic effects of pumiliotoxin B are reversed only by relatively high concentrations of the calcium channel blockers nifedipine and verapamil, suggesting that pumiliotoxin B may owe its cardiotonic activities to effects on internal mobilization of calcium.

Structural prerequisites for the hypotensive action of parathyroid hormone

We assessed the vascular, phosphaturic, and calcemic responses to several synthetic parathyroid hormone (PTH) analogues. Bovine (b) PTH (1-34), human (h) PTH (1-34), hPTH (53-84), [ Nle8 , Nle18 , Tyr34 ]bPTH (1-34), and [ Nle8 , Nle18 , Tyr34 ]bPTH (3-34) were administered in doses between 1 and 500 micrograms/kg as bolus intravenous injections to male Wistar-Kyoto rats aged 18-26 wk. Antagonism of the action of PTH was assessed in rats pretreated with 10 or 100 micrograms/kg [ Nle8 , Nle18 , Tyr34 ]bPTH (3-34) followed by 10 micrograms/kg of bPTH (1-34), or with 10 micrograms/kg hPTH (53-84) followed by 10 micrograms/kg hPTH (1-34). Bovine PTH (1-34), hPTH (1-34), and [ Nle8 , Nle18 , Tyr34 ]bPTH (1-34) produced virtually identical log dose-dependent hypotension, with 100 micrograms/kg of each analogue producing a 56% reduction in mean arterial pressure. Neither hPTH (53-84) nor [ Nle8 , Nle18 , Tyr34 ]bPTH (3-34) demonstrated any effect on mean arterial pressure at doses up to 500 micrograms/kg. Pretreatment with the inactive analogues failed to antagonize the vasodilating response to either bPTH (1-34) or hPTH (1-34). The vasoactive analogues significantly increased urinary phosphorus excretion while the inactive analogues did not modify it. hPTH (1-34) produced a modest decrease in serum Ca2+ at 1 min after injection. The results document that the vasodilating effect of PTH is a specific action of the peptide. Deletion of the first two amino acid residues abolishes both the phosphaturic and hypotensive effects of the peptide. Acute changes in serum Ca2+ do not appear to be a prerequisite for the vasodilatory response. Inactive analogues of PTH do not antagonize the vascular actions of the peptide.

Vasodilation mediated by human PTH 1-34 in the spontaneously hypertensive rat

Parathyroid hormone's cardiovascular effects were assessed in a model of experimental hypertension with known abnormalities of calcium metabolism. Mean arterial pressure (MAP) changes and serum ionized calcium responses were measured in the spontaneously hypertensive rat (SHR) and its normotensive control, the Wistar-Kyoto (WKY), following injections of synthetic human PTH 1-34. Six 22-wk-old SHR and six WKY were given intra-arterial serial injections (0.1-100 micrograms/kg) of hPTH 1-34. Both the SHR (P less than 0.001) and WKY (P less than 0.001) demonstrated log dose-dependent hypotensive responses that were maximal at 1 min, with recovery occurring between 15 and 30 min. The slopes, however, of the dose-response curves differed (P less than 0.01). The SHR experienced a greater maximal delta MAP [-93.7 +/- 2.4 (SHR) vs. -71.2 +/- 1.6 mmHg (WKY), P less than 0.01]. Furthermore, the duration of the hypotensive action of hPTH 1-34 was significantly longer (P less than 0.001) in the SHR. Even when corrected for base-line MAP the SHR demonstrated a significant (P = 0.025) enhancement of this vasodilator response at doses of 5 micrograms/kg and greater at time intervals between 3 and 9 min after injection. A transient decrease [2.25 +/- 0.10 (pre) vs. 2.17 +/- 0.11 meq/liter (1 min post), P less than 0.01] in serum ionized calcium occurred at 1 min. We conclude that hPTH 1-34 is a potent vasoactive peptide in both the normotensive WKY and the SHR. The greater maximal hypotensive response to hPTH 1-34 and the prolongation of this cardiovascular effect in the SHR may be an additional manifestation of this experimental animal's acknowledged abnormalities of cellular membrane calcium and phospholipid metabolism.

Infusion of bovine parathyroid hormone1-34 attenuates the pressor response to angiotensin II in spontaneously hypertensive rats

Parathyroid hormone, long known to be important for calcium homeostasis, also has potent vascular effects. In the past, pharmacologic doses of parathyroid hormone or its active amino-terminal fragment (PTH1-34) were necessary to demonstrate vasoactivity. We assessed the vascular effects of physiologic doses of infused synthetic bovine PTH1-34 (15 U/hr or 1.5 micrograms/hr) on the pressor response to angiotensin II. PTH1-34 attenuated the pressor response (p less than 0.005) to one to 100 nanograms of angiotensin II in both Aoki-Okamoto spontaneously hypertensive and normotensive Wistar Kyoto rats. There was no difference in response to either PTH or angiotensin II between strains, and at the lower doses of angiotensin II, PTH1-34 dampened the pressor response by as much as 33%. These results suggest that endogenous PTH may modulate systemic blood pressure and regional vascular resistance. In various states of parathyroid gland stimulation, the peptide may exert physiologically important vascular effects.