Distinct effects of CGRP on typical and atypical smooth muscle cells involved in generating spontaneous contractions in the mouse renal pelvis

Background and purpose:

We investigated the cellular mechanisms underlying spontaneous contractions in the mouse renal pelvis, regulated by calcitonin gene-related peptide (CGRP).

Experimental approach:

Spontaneous contractions, action potentials and Ca²⁺ transients in typical and atypical smooth muscle cells (TSMCs and ATSMCs) within the renal pelvis wall were recorded separately using tension and intracellular microelectrode recording techniques and Fluo-4 Ca²⁺ imaging. Immunohistochemical and electron microscopic studies were also carried out.

Key results:

Bundles of CGRP containing transient receptor potential cation channel, subfamily V, member 1-positive sensory nerves were situated near both TSMCs and ATSMCs. Nerve stimulation reduced the frequency but augmented the amplitude and duration of spontaneous phasic contractions, action potentials and Ca²⁺ transients in TSMCs. CGRP and agents increasing internal cyclic adenosine monophosphate (cAMP) mimicked the nerve-mediated modulation of TSMC activity and suppressed ATSMCs Ca²⁺ transients. Membrane hyperpolarization induced by CGRP or cAMP stimulators was blocked by glibenclamide, while their negative chronotropic effects were less affected. Glibenclamide enhanced TSMC Ca²⁺ transients but inhibited ATSMC Ca²⁺ transients, while both 5-hydroxydecanoate and diazoxide, a blocker and opener of mitochondrial ATP-sensitive K⁺ channels, respectively, reduced the Ca²⁺ transient frequency in both TSMCs and ATSMCs. Inhibition of mitochondrial function blocked ATSMCs Ca²⁺ transients and inhibited spontaneous excitation of TSMCs.

Conclusions and implications:

The negative chronotropic effects of CGRP result primarily from suppression of ATSMC Ca²⁺ transients rather than opening of plasmalemmal ATP-sensitive K⁺ channels in TSMCs. The positive inotropic effects of CGRP may derive from activation of TSMC L-type Ca²⁺ channels. Mitochondrial Ca²⁺ handling in ATSMCs also plays a critical role in generating Ca²⁺ transients.