Angiotensin II type 2 receptor inhibits expression and function of insulin receptor in rat renal proximal tubule cells

Insulin enhances renal glucose excretion: relation to insulin sensitivity and sodium-glucose cotransport

INTRODUCTION

Insulin regulates renal glucose production and utilization; both these fluxes are increased in type 2 diabetes (T2D). Whether insulin also controls urinary glucose excretion is not known.

METHODS

We applied the pancreatic clamp technique in 12 healthy subjects and 13 T2D subjects. Each participant received a somatostatin infusion and a variable glucose infusion to achieve (within 1 hour) and maintain glycemia at 22 mmol/L for 3 hours; next, a constant insulin infusion (240 pmol/min/kg) was added for another 3 hours. Urine was collected separately in each period for glucose and creatinine determination.

RESULTS

During saline, glucose excretion was lower in T2D than controls in absolute terms (0.49 (0.32) vs 0.69 (0.18) mmol/min, median (IQR), p=0.01) and as a fraction of filtered glucose (16.2 (6.4) vs 19.9 (7.5)%, p<0.001). With insulin, whole-body glucose disposal rose more in controls than T2D (183 (48) vs 101 (48) µmol/kg_FFM/min, p<0.0003). Insulin stimulated absolute and fractional glucose excretion in controls (p<0.01) but not in T2D. Sodium excretion paralleled glucose excretion. In the pooled data, fractional glucose excretion was directly related to whole-body glucose disposal and to fractional sodium excretion (r=0.52 and 0.54, both p<0.01). In another group of healthy controls, empagliflozin was administered before starting the pancreatic clamp to block sodium-glucose cotransporter 2 (SGLT2). Under these conditions, insulin still enhanced both glucose and sodium excretion.

CONCLUSIONS

Acute exogenous insulin infusion jointly stimulates renal glucose and sodium excretion, indicating that the effect may be mediated by SGLTs. This action is resistant in patients with diabetes, accounting for their increased retention of glucose and sodium, and is not abolished by partial SGLT2 inhibition by empagliflozin.

The Other Angiotensin II Receptor: AT2R as a Therapeutic Target

The active hormone of the renin-angiotensin system (RAS), angiotensin II (Ang II), is involved in several human diseases, driving the development and clinical use of several therapeutic drugs, mostly angiotensin I converting enzyme (ACE) inhibitors and angiotensin receptor type I (AT₁R) antagonists. However, angiotensin peptides can also bind to receptors different from AT₁R, in particular, angiotensin receptor type II (AT₂R), resulting in biological and physiological effects different, and sometimes antagonistic, of their binding to AT₁R. In the present Perspective, the components of the RAS and the therapeutic tools developed to control it will be reviewed. In particular, the characteristics of AT₂R and tools to modulate its functions will be discussed. Agonists or antagonists to AT₂R are potential therapeutics in cardiovascular diseases, for agonists, and in the control of pain, for antagonists, respectively. However, controlling their binding properties and their targeting to the target tissues must be optimized.

Effect of Insulin on Proximal Tubules Handling of Glucose: A Systematic Review

Renal proximal tubules reabsorb glucose from the glomerular filtrate and release it back into the circulation. Modulation of glomerular filtration and renal glucose disposal are some of the insulin actions, but little is known about a possible insulin effect on tubular glucose reabsorption. This review is aimed at synthesizing the current knowledge about insulin action on glucose handling by proximal tubules. Method. A systematic article selection from Medline (PubMed) and Embase between 2008 and 2019. 180 selected articles were clustered into topics (renal insulin handling, proximal tubule glucose transport, renal gluconeogenesis, and renal insulin resistance). Summary of Results. Insulin upregulates its renal uptake and degradation, and there is probably a renal site-specific insulin action and resistance; studies in diabetic animal models suggest that insulin increases renal SGLT2 protein content; in vivo human studies on glucose transport are few, and results of glucose transporter protein and mRNA contents are conflicting in human kidney biopsies; maximum renal glucose reabsorptive capacity is higher in diabetic patients than in healthy subjects; glucose stimulates SGLT1, SGLT2, and GLUT2 in renal cell cultures while insulin raises SGLT2 protein availability and activity and seems to directly inhibit the SGLT1 activity despite it activating this transporter indirectly. Besides, insulin regulates SGLT2 inhibitor bioavailability, inhibits renal gluconeogenesis, and interferes with Na⁺K⁺ATPase activity impacting on glucose transport. Conclusion. Available data points to an important insulin participation in renal glucose handling, including tubular glucose transport, but human studies with reproducible and comparable method are still needed.

Plantago asiatica Seed Extracts Alleviated Blood Pressure in Phase I⁻Spontaneous Hypertension Rats

Arterial pressure of each new breeding spontaneous Phase-1 hypertension (P1-HT) rat was recorded for 5 min by intravascular femoral artery catheter that served as a reference value prior to treatment. In the acute antihypertensive test, 0.36 g/kg Bwt of Plantago asiatica seed extract (PSE) was administered, via gavage feeding, to P1-HT rats, and the arterial pressures were continuously recorded for 1 h. The acute antihypertensive effects of PSE on P1-HT rats appeared within 15 min after PSE administration and lasted over 1 h with systolic pressure decreased 31.5 mmHg and diastolic pressure decreased 18.5 mmHg. The systolic pressure decreased 28 mmHg and diastolic pressure decreased 16 mmHg in P1-HT rats when simultaneously compared with verapamil hydrochloride (reference drug), whereas there were no significant differences in the pretreated reference values of acute PSE treatment and the untreated control. In the chronic test, P1-HT rats received 0.36 g/kg Bwt day of PSE or equal volume of water for 4 weeks via oral gavage, and the lower blood pressure tendencies of chronic PSE treatment were also found when compared with the controls. The antihypertensive values of PSE were also confirmed in spontaneously hypertensive rats (SHRs). Oral administration with PSE can effectively moderate blood pressure within an hour, while taking PSE daily can control the severity of hypertension, suggesting PSE is a potentially antihypertensive herb.