What effect do increased renin and angiotensin II (Angiotensin 2) levels have on the vascular tone of the glomerular afferent arteriole in the setting of hypovolemia (volume depletion)?

From the Guidelines

In the setting of volume depletion, increased renin and angiotensin II levels cause preferential constriction of the efferent arteriole while having minimal effect on the afferent arteriole, which is crucial for maintaining glomerular filtration rate (GFR) during hypovolemia. This differential effect is due to the counterbalancing vasodilatory effects of prostaglandins and nitric oxide that are locally produced in response to reduced renal perfusion, as well as the afferent arteriole having fewer angiotensin II receptors compared to the efferent arteriole 1. The mechanism by which angiotensin II helps maintain glomerular hydrostatic pressure and GFR despite decreased renal blood flow is an important physiological adaptation to preserve kidney function during states of volume depletion, ensuring continued filtration and urine production even when systemic blood pressure is reduced. Some key points to consider in this context include:

• The afferent arteriole is relatively protected from angiotensin II-mediated vasoconstriction due to the local production of vasodilatory substances 1.
• The efferent arteriole has more angiotensin II receptors and is more sensitive to the vasoconstrictive effects of angiotensin II, which helps maintain glomerular hydrostatic pressure and GFR 1.
• The use of ACE inhibitors can affect this balance and may lead to a decrease in GFR, particularly in patients with volume depletion or bilateral renal artery stenosis 1.
• Recent guidelines suggest continuing renin-angiotensin system blockade for mild to moderate increases in serum creatinine (≤30%) in individuals who have no signs of extracellular fluid volume depletion 1.

From the Research

Vascular Tone of the Glomerular Afferent Arteriole

• In the setting of hypovolemia, increased renin and angiotensin II levels have a vasoconstrictor effect on the glomerular afferent arteriole 2.
• Angiotensin II causes vasoconstriction at low concentrations, which can lead to a decrease in afferent arteriolar diameter 3, 2.
• The vasoconstrictor effect of angiotensin II on the afferent arteriole is mediated by the release of calcium from intracellular stores 2.
• However, it is also suggested that prorenin (renin I) can generate localized high concentrations of angiotensin II, causing regional dilation of the afferent arteriole 3.

Regulation of Glomerular Filtration Rate

• The renin-angiotensin system plays a crucial role in regulating glomerular filtration rate (GFR) and maintaining blood pressure homeostasis 4, 5.
• Angiotensin II constricts the efferent arteriole, which helps to maintain GFR in circumstances associated with impaired renal perfusion 4, 5.
• The constrictor effect of angiotensin II on the efferent arteriole is important in preventing reductions in GFR, especially in conditions such as renal artery stenosis 5, 6.

Role of Angiotensin II in Renovascular Hypertension

• Angiotensin II maintains GFR in renovascular hypertension by constricting the efferent arteriole and increasing the glomerular filtration fraction 6.
• The homeostatic action of angiotensin II to maintain GFR has minimal effects on blood flow, but it plays a crucial role in regulating renal resistance and glomerular ultrafiltration 6.