This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
According to myogenic hypothesis of renal  autoregulation,  the  afferent  arterioles contract in response to stretch induced by 
Select and explain. 
A. NO release, B. Noradrenaline release, C. Opening of Ca2+  channels, D. Adenosine release
C: Opening of Ca2+  channels
Answer: C: Opening of Ca2+  channels 
Explanation: Stretch on smooth muscle (afferent  arterioles) opens Ca+ channel, which causes contraction of afferent arterioles.the medullary interstitium.
REGULATION OF GFR
Regulation is done by:

Blood pressure: Autoregulation
Resistance of afferent and efferent anterioles

- Autoregulation: Constant maintenance of renal blood flow or, GFR over the span of renal perfusion pressure (pressure in renal artery minus pressure in renal vein ) from 80 to about 170mm Hg
- Mechanism of autoregulation:
i) Myogenic mechanism- contraction or relaxation of anteriolar smooth muscle in response to changes in vascular pressures; mediated by Ca++
ii) Tubule glomerular feedback (TGF) mechanism-mediated by macula densa cells (described later)