According to myogenic hypothesis of renal autoregulation, the afferent aerioles contract due to –
**Core Concept**
The myogenic hypothesis of renal autoregulation proposes that changes in blood pressure lead to contraction or relaxation of renal afferent arterioles, maintaining a relatively constant glomerular filtration rate (GFR) despite fluctuations in systemic blood pressure. This mechanism is crucial for maintaining renal function in the face of varying systemic pressures.
**Why the Correct Answer is Right**
The myogenic hypothesis is mediated by the stretch-sensitive ion channels in the smooth muscle cells of the afferent arterioles. When blood pressure increases, the afferent arterioles stretch, leading to depolarization of the smooth muscle cells and contraction of the arteriole. This contraction increases vascular resistance, which in turn helps to maintain a constant GFR. The stretch-sensitive ion channels involved in this process are primarily the mechanically gated calcium channels, which allow an influx of calcium ions into the smooth muscle cells, triggering contraction.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the myogenic hypothesis is not primarily mediated by the renin-angiotensin-aldosterone system (RAAS) or the sympathetic nervous system. While these systems do play a role in renal autoregulation, they are not the primary mechanism described by the myogenic hypothesis.
**Option B:** This option is incorrect because the tubuloglomerular feedback mechanism, while an important component of renal autoregulation, is a separate mechanism from the myogenic hypothesis. Tubuloglomerular feedback is mediated by changes in sodium delivery to the distal tubules, which trigger a feedback response to adjust GFR.
**Option C:** This option is incorrect because the juxtaglomerular apparatus (JGA) is primarily involved in the RAAS and is not directly related to the myogenic hypothesis.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the myogenic hypothesis is a local, intrinsic mechanism that helps to maintain renal autoregulation, whereas the RAAS and sympathetic nervous system are extrinsic mechanisms that also play a role in regulating renal function.
**Correct Answer: A. Stretch-sensitive ion channels in the smooth muscle cells of the afferent arterioles.**