Smooth muscle-selective deletion of guanylyl cyclase-A prevents the acute but not chronic effects of ANP on blood pressure

Atrial natriuretic peptide (ANP) is an important regulator of arterial blood pressure. The mechanisms mediating its hypotensive effects are complex and involve the inhibition of the sympathetic and renin-angiotensin-aldosterone (RAA) systems, increased diuresis/natriuresis, vasodilation, and enhanced vascular permeability. In particular, the contribution of the direct vasodilating effect of ANP to the hypotensive actions remains controversial, because variable levels of the ANP receptor, guanylyl cyclase A (GC-A), are expressed in different vascular beds. The objective of our study was to determine whether a selective deletion of GC-A in vascular smooth muscle would affect the hypotensive actions of ANP. We first created a mutant allele of mouse GC-A by flanking a required exon with loxP sequences. Crossing floxed GC-A with SM22-Cre transgene mice expressing Cre recombinase in smooth muscle cells (SMC) resulted in mice in which vascular GC-A mRNA expression was reduced by ≈80%. Accordingly, the relaxing effects of ANP on isolated vessels from these mice were abolished; despite this fact, chronic arterial blood pressure of awake SMC GC-A KO mice was normal. Infusion of ANP caused immediate decreases in blood pressure in floxed GC-A but not in SMC GC-A knockout mice. Furthermore, acute vascular volume expansion, which causes release of cardiac ANP, did not affect resting blood pressure of floxed GC-A mice, but rapidly and significantly increased blood pressure of SMC GC-A knockout mice. We conclude that vascular GC-A is dispensable in the chronic and critical in the acute moderation of arterial blood pressure by ANP.

心房钠尿肽（Atrial natriuretic peptide, ANP）是动脉血压的重要调节因子。其介导降压效应的机制较为复杂，涉及抑制交感神经及肾素-血管紧张素-醛固酮（renin-angiotensin-aldosterone, RAA）系统、增强利尿/利钠作用、舒张血管以及提升血管通透性。其中，ANP直接血管舒张效应对其降压作用的贡献仍存在争议，原因是不同血管床中ANP受体鸟苷酸环化酶A（guanylyl cyclase A, GC-A）的表达水平存在差异。本研究的目的是明确血管平滑肌中GC-A的选择性敲除是否会影响ANP的降压作用。我们首先通过在必需外显子两侧插入loxP序列，构建了小鼠GC-A的突变等位基因。将带loxP位点的GC-A与在平滑肌细胞（smooth muscle cells, SMC）中表达Cre重组酶的SM22-Cre转基因小鼠交配，获得了血管GC-A mRNA表达水平降低约80%的小鼠。相应地，ANP对这些小鼠离体血管的舒张效应被完全消除；尽管如此，清醒状态下的平滑肌细胞GC-A敲除（knockout, KO）小鼠的慢性动脉血压仍处于正常水平。向带loxP位点的GC-A小鼠输注ANP可快速降低其血压，但平滑肌细胞GC-A敲除（KO）小鼠无此变化。此外，可触发心脏分泌ANP的急性血管容量扩张，对带loxP位点的GC-A小鼠的基础血压无影响，但会快速且显著升高平滑肌细胞GC-A敲除（KO）小鼠的血压。综上，我们认为血管GC-A在ANP介导的动脉血压慢性调节中并非必需，而在急性调节中至关重要。