Prostaglandin E2 inhibits Na-K-2Cl cotransport in medullary thick ascending limb cells

Opportunities for the treatment of erectile dysfunction by modulation of the NO axis--alternatives to sildenafil citrate

Erectile function in man depends upon a complex interaction of psychogenic, neurologic, hormonal and vascular factors, and therefore the management of erectile dysfunction (ED) reflects this complexity of control. Therapeutic options include psychological and non-pharmacological approaches as well as drug treatments. The effectiveness of the type-5 cGMP phosphodiesterase inhibitor sildenafil citrate (Viagra) confirms the pivotal role of the NO-cGMP axis in promoting and maintaining erection. Although widely acclaimed, sildenafil leaves many questions unanswered, especially regarding its susceptibility to pharmacokinetic drug interactions, and its safety in patients with ischaemic heart disease and those taking nitrates. In view of the epidemiological link between erectile dysfunction and cardiovascular disease in the elderly, this limitation might have much broader implications. The presently available scientific documentation, although less extensive, indicates that NO donors, such as topically applied nitroglycerin (GTN; for example, 1-2 puffs of an ordinary GTN spray applied to the shaft of the penis), might be a reasonable alternative. Further larger-scale research on the efficacy and tolerability of topical GTN is needed to establish its full therapeutic potential in the treatment of erectile dysfunction.

Angiotensin II induces TNF production by the thick ascending limb: functional implications

The effects of angiotensin II (ANG II) on tumor necrosis factor-alpha (TNF) production were determined in freshly isolated tubules from the medullary thick ascending limb (MTAL). ANG II (10(-9) M) increased the accumulation of TNF mRNA associated with enhanced production of TNF by approximately five- to sixfold. ANG II also increased prostaglandin E2 (PGE2) production by the MTAL in a dose-dependent manner and exerted biphasic differential effects on 86Rb uptake, depending on the exposure time of the tubules to the peptide and the doses used. Low-dose ANG II (10(-11) M) increased 86Rb uptake by MTAL tubules after a "short-term" (15 min) challenge, whereas uptake was inhibited after a "long-term" (3 h) incubation period. High-dose ANG II (10(-6) M) inhibited MTAL 86Rb uptake, irrespective of incubation time. Uptake of 86Rb was inhibited by approximately 60% in MTAL tubules that were challenged for 3 h with ANG II. The inhibitory action of ANG II was prevented by eliminating the participation of either TNF with antisera to the cytokine or PGE2 by inhibition of cyclooxygenase with indomethacin. We conclude that ANG II regulates TNF production in the MTAL, an interaction that affects 86Rb uptake via an eicosanoid-dependent mechanism in this nephron segment.

Hypercalcemia stimulates expression of intrarenal phospholipase A2 and prostaglandin H synthase-2 in rats. Role of angiotensin II AT1 receptors

In chronic hypercalcemia, inhibition of thick ascending limb sodium chloride reabsorption is mediated by elevated intrarenal PGE2. The mechanisms and source of elevated PGE2 in hypercalcemia are not known. We determined the effect of hypercalcemia on intrarenal expression of cytosolic phospholipase A2 (cPLA2), prostaglandin H synthase-1 (PGHS-1), and prostaglandin H synthase-2 (PGHS-2), enzymes important in prostaglandin production. In rats fed dihydrotachysterol to induce hypercalcemia, Western blot analysis revealed significant upregulation of both cPLA2 and PGHS-2 in the kidney cortex and the inner and outer medulla. Immunofluorescence localized intrarenal cPLA2 and PGHS-2 to interstitial cells of the inner and outer medulla, and to macula densa and cortical thick ascending limbs in both control and hypercalcemic rats. Hypercalcemia had no effect on intrarenal expression of PGHS-1. To determine if AT1 angiotensin II receptor activation was involved in the stimulation of cPLA2 and PGHS-2 in hypercalcemia, we treated rats with the AT1 receptor antagonist, losartan. Losartan abolished the polydipsia associated with hypercalcemia, prevented the increase in cPLA2 protein in all regions of the kidney, and diminished PGHS-2 expression in the inner medulla. In addition, losartan completely prevented the increase in urinary PGE2 excretion in hypercalcemic rats. Intrarenal levels of angiotensin II were unchanged in hypercalcemia. These data indicate that hypercalcemia stimulates intrarenal cPLA2 and PGHS-2 protein expression. Our results further support a role for angiotensin II, acting on AT1 receptors, in mediating this stimulation.

Effects of long-term oral administration of L-arginine on the rat erectile response

PURPOSE

Nitric oxide (NO), the neurotransmitter responsible for mediating penile erection in the rat, is synthesized from L arginine by nitric oxide synthase (NOS) in a reaction blocked by L-NAME (N-omega-nitro-L-arginine methyl ester). To determine whether dietary supplementation of L-arginine can stimulate penile erection and whether ancillary pathways for penile erection may exist, a series of experiments were conducted in the Fischer 344 rat.

MATERIALS AND METHODS

Adult male (5 month old) and aged (20 month old) rats were fed L-arginine (2.25%) and L-NAME (0.7%) dissolved in tap water for 8 weeks. Animals (n = 6) underwent electrical field stimulation (EFS) of the cavernosal nerve to induce erection and both maximal intracavernosal pressure (MIP) and mean arterial pressure (MAP, mm. Hg +/- SEM) were measured. Tissue and serum levels of L-arginine were measured by an automated amino acid analyzer. Penile eNOS (endothelial) and nNOS (neuronal) content were measured by western blot densitometry. Total penile NOS enzyme activity was measured by the L-arginine to L-citrulline conversion assay.

RESULTS

The L-arginine fed animals demonstrated a significant increase in EFS-induced MIP when compared to the controls in both the adult (104 +/- 4 vs. 86 +/- 6, p = 0.04) and aged (87 +/- 5 vs. 66 +/- 4, p = 0.02) animals, without changes in MAP. L-NAME virtually abolished the MIP in adult rats (8 +/- 3, p < 0.0001), while increasing the MAP (186 +/- 8, p < 0.0001). Serum and penile tissue levels of L-arginine were increased by 64-148% in all groups compared to control animals. Penile eNOS and nNOS content remained unchanged in control and treated animals. Penile NOS activity was increased nearly 100% in the L-arginine treated groups vs. controls.

CONCLUSIONS

Long-term oral administration of supra-physiologic doses of L-arginine improves the erectile response in the aging rat. We postulate that L-arginine in the penis may be a substrate-limiting factor for NOS activity and that L-arginine may up-regulate penile NOS activity but not its expression. The blockade of penile erection by EFS with L NAME suggests that if ancillary corporeal vasodilator mechanisms develop, a basal level of NO synthesis is still required for activation and relaxation of the corporeal smooth muscle. These data support the possible use of dietary supplements for treatment of erectile dysfunction.