Is there any service available in designer through which we reload the trustore?
you can investigate the DSP source code of the Keystore page in IS Admin where the reload button is located for the action being invoked when the reload button is clicked.
Most likely this would refer to an internal service located in WmRoot package which is hidden to Designer by default.
I would suggest you to check out the Admin API at <host: port>/admin/swagger/integrationServer. There is an option to reload the truststore via the admin API on the lines of /admin/truststore/ starting with 10.7.
The WmRoot services can change, but the admin API is expected to be consistent.
As Nagendra suggested, have a look at the IS Admin API. It was introduced in v10.5.
This article gives some info too Integration server performing management actions from CLI
There is also info in the IS admin guide, and a Postman collection you can download from this tech community.