Tuning Electrochemical Stability of 5,10-Ditolylphenazine-Based Antiaromatic Materials for Unipolar Memristor toward Artificial Synapses Application

Three organic conjugated small molecules, DTA-DTPZ, Cz-DTPZ, and DTA-me-DTPZ comprising an antiaromatic 5,10-ditolylphenazine (DTPZ) core and electron-donating peripheral substituents with high HOMOs (-4.2 to -4.7 eV) and multiple reversible oxidative potentials are reported. The corresponding films sandwiched between two electrodes show unipolar and switchable hysteresis current-voltage (I-V) characteristics upon voltage sweeping, revealing the prominent features of nonvolatile memristor behaviors. The numerical simulation of the I-V curves suggests that the carriers generated by the oxidized molecules lead to the increment of conductance. However, the accumulated carriers tend to deteriorate the device endurance. The electroactive sites are fully blocked in the dimethylated molecule DTA-me-DTPZ, preventing the irreversible electrochemical reaction, thereby boosting the endurance of the memristor device over 300 cycles. Despite the considerable improvement in endurance, the decrement of on/off ratio from 105 to 101 after 250 cycles suggests that the excessive charge carriers (radical cations) remains a problem. Thus, a new strategy of doping an electron-deficient material, CN-T2T, into the unipolar active layer was introduced to further improve the device stability. The device containing DTA-me-DTPZ:CNT2T (1:1) blend as the active layer retained the endurance and on/off ratio (∼104) upon sweeping 300 cycles. The molecular designs and doping strategy demonstrate effective approaches toward more stable metal-free organic conjugated small-molecule memristors.

Effect of blockade of nitric oxide synthesis on renin secretion in human subjects

Nitric oxide (NO) has been implicated in the control of renin secretion in experimental animals but little information is available concerning its role in humans. The aim of the present study was to investigate the effects of inhibition of NO synthesis on resting renin secretion and on the renin secretory responses to activation of the macula densa and sympathetic neural mechanisms controlling renin secretion. In eight healthy subjects, injection of furosemide increased plasma renin activity (PRA) with little or no change in blood pressure or heart rate. Injection of the NO synthase inhibitor L-NMMA increased blood pressure and decreased heart rate and PRA, but failed to alter the PRA response to furosemide. In another ten subjects, standing increased PRA. L-NMMA again decreased PRA but failed to alter the PRA response to standing. These results suggest that NO participates in the regulation of resting renin secretion in humans, and provide preliminary evidence that NO does not contribute significantly to the renin responses to activation of the macula densa or sympathetic mechanisms controlling renin secretion.

Inhibition of phosphodiesterase III with milrinone increases renin secretion in human subjects

One of the major signaling molecules involved in the regulation of renin secretion is cyclic AMP (cAMP). The concentration of cAMP in cells is determined in part by the rate of cAMP hydrolysis by several families of phosphodiesterases, especially the phosphodiesterase III family, but little is known about the roles of these enzymes in the control of renin secretion, particularly in humans. The aim of the present study was to investigate the effect of the phosphodiesterase III inhibitor milrinone on renin secretion in human subjects. Milrinone was infused i.v. in eight healthy normotensive subjects in a dose of 100 microgram/kg. Immediately after the infusion, there was a transient increase in systolic pressure from 107 +/- 5 to 116 +/- 5 mm Hg (p <.01), but no significant change in diastolic or mean arterial pressure. Heart rate increased from 67 +/- 2 to 86 +/- 4 beats/min (p <.01) and remained elevated. Plasma renin activity increased in all subjects, the mean value increasing from 3.0 +/- 0.5 to 6.0 +/- 1.1 ng/ml/h at 15 min (p <.01). These results demonstrate that milrinone increases renin secretion in human subjects, thus providing evidence that phosphodiesterase III family participates in the control of renin secretion in humans. The increase in renin secretion does not appear to be mediated by major mechanisms that control renin secretion, and likely results from an increase in cAMP concentration in the juxtaglomerular cells.

Cardiovascular and endocrine effects of acupuncture in hypertensive patients

It has been reported that acupuncture can decrease blood pressure in patients with hypertension, possibly by an endocrine mechanism. The aim of the present study was to investigate the effects of acupuncture on arterial blood pressure and the secretion of renin, vasopressin and cortisol in hypertensive patients. Acupuncture was performed in fifty untreated essential hypertensive patients resting in the supine position. Thirty min after acupuncture there were decreases in systolic pressure from 169 +/- 2 to 151 +/- 2 mm Hg, diastolic pressure from 107 +/- 1 to 96 +/- 1 mm Hg, and heart rate from 77 +/- 2 to 72 +/- 2 bpm (P < 0.01). Plasma renin activity decreased from 1.7 +/- 0.4 to 1.1 +/- 0.2 ng/ml/2h (P < 0.01), but there were no significant changes in plasma vasopressin or cortisol concentrations. These results confirm that acupuncture decreases blood pressure in hypertensive patients, and suggest that the decrease results, at least in part, from a decrease in renin secretion.

Nitric oxide and the control of renin secretion

Research during recent years has established nitric oxide as a unique signaling molecule that plays important roles in the regulation of the cardiovascular, nervous, renal, immune and other systems. Nitric oxide has also been implicated in the control of the secretion of hormones by the pancreas, hypothalamus, pituitary and other endocrine glands, and evidence is accumulating that it contributes to the regulation of the secretion of renin by the kidneys. The enzyme nitric oxide synthetase is present in vascular and tubular elements of the kidney, particularly in cells of the macula densa, a structure that plays an important role in the control of renin secretion. Guanylyl cyclase, a major target for nitric oxide, is also present in the kidney and is responsive to changes in nitric oxide levels. Drugs that inhibit nitric oxide synthesis generally suppress renin release in vivo and in vitro, suggesting a stimulatory role for the L-arginine-nitric oxide pathway in the control of renin secretion. Under some conditions, however, blockade of nitric oxide synthesis increases renin secretion. Recent studies indicate that nitric oxide not only contributes to the regulation of basal renin secretion, but also participates in the renin secretory responses to activation of the renal baroreceptor, macula densa and beta adrenoceptor mechanisms that regulate renin secretion. Future research should clarify the mechanisms by which nitric oxide regulates the secretion of renin and establish the physiological significance of this regulation.